GFF3 format

Supports all annotated assemblies

GFF3 format

Supports all annotated assemblies

Genome annotation files are provided in GFF3 format for all annotated assemblies included in NCBI’s Genomes FTP resource.
GFF3 files are formatted according to the specifications published by the Sequence Ontology.

NCBI’s GFF3 files differ from the official GFF3 specifications for certain attributes and formatting details.

Directives

Official directives

The following directives or pragmas are provided according to the GFF3 specifications:

##gff-version

##sequence-region

##species

Unofficial directives

The following directives are not part of the official GFF3 specifications and have been proposed or are provided as additional information to aid in parsing the files. They are indicated by a leading ‘#!’.

#!gff-spec-version
specific version of the GFF3 specification that the files conform to

#!processor
NCBI application used to generate the files

#!genome-build
Assembly name, if the set of seqids included in the file correspond to a specific genome assembly

#!genome-build-accession
Assembly accession.version as defined in NCBI’s Assembly resource. This is a stable, unique identifier for the set of sequence records included in an assembly as found in INSDC or RefSeq, and it is highly recommended to use the assembly accession.version in addition to or instead of the assembly name to ensure consistency in datasets.

#!annotation-date
Date the annotation was produced, if available

#!annotation-source
Source and name to use for the annotation, if available

Column Specifications

Column 1: “seqid”
Accession.version of the annotated genomic sequence. NCBI files universally use accession.version because it provides an unambiguous identifier for the annotated sequence, and does not require additional knowledge of the species, assembly and version, and data source. We strongly recommend using accession.version instead of ambiguous seqids such as ‘chr1’ to avoid errors due to mis-associating features with the wrong genomic location.

Column 2: “source”
For annotations produced by one of NCBI’s pipelines, the method used to generate the annotation is provided in column 2. The method is found in the ModelEvidence object in ASN.1 format, and appears in the flatfile format as a structured note. For example: “Derived by automated computational analysis using gene prediction method: BestRefSeq”

The reported methods for RefSeq eukaryotic annotations include:

BestRefSeq: feature projected from the alignment of a “known” RefSeq transcript to the genome

Curated Genomic: feature projected from the alignment of a curated RefSeq genomic sequence to the genome

Gnomon: feature predicted by Gnomon, using transcript and protein evidence and/or ab initio

BestRefSeq,Gnomon: gene with children features predicted by BestRefSeq and Gnomon

Curated Genomic,Gnomon: gene with children features predicted Curated Genomic or Gnomon

tRNAscan-SE: feature predicted by tRNAscan-SE

The reported methods for RefSeq prokaryotic annotations include:

GeneMarkS+: feature predicted by GeneMarkS+

Protein Homology: feature predicted by protein alignment

cmsearch: feature predicted by cmsearch

tRNAscan-SE: feature predicted by tRNAscan-SE

If the annotation method is not available, the source column is based on the source database for the record (RefSeq, GenBank, EMBL, DDBJ).

Column 3: “type”
The SOFA feature type most equivalent to the feature found in the source annotation. The original GenBank feature type is also provided by the “gbkey” attribute in column 9.

Columns 4 & 5: “start” and “end”
Start and end coordinates of the feature in 1-based coordinates. Note two exon or CDS rows of the same feature may overlap or be separated by an artificial “micro-intron” in order to represent cases of ribosomal slippage or putative assembly errors. See Additional Details below for more information.

Column 6: “score”
Currently only provided for alignments, if they contain a score named “score”. The definition of this score may vary depending on the type of alignment.

Column 7: “strand”
The strand of the feature

Column 8: “phase”
The phase of the CDS feature, which is related to /codon_start in the flatfile specification. The phase is computed based on the known phase at the start of the CDS and computed for subsequent CDS rows. It may not be accurate if the CDS contains internal frameshifts, which can occur in pseudogenes and in genomes with indels, assembly gaps, and other errors. See Additional Details below for more information.

Column 9: “attributes”
A semicolon delimited list of official and additional attributes describing the feature.

Attribute Specifications

Attributes in column 9 conform to the GFF3 specification, which uses a starting Capital letter for official attributes and starting lower-case letter for other attributes.

Official GFF3 attributes

ID
A unique identifier for the feature. Most IDs are generated on-the-fly during file generation. They are not intended to be used as stable feature identifiers, and they are likely to change between annotation versions. Multiple rows with the same ID designate a single feature that is composed of multiple parts, most common for CDSes and multi-exon alignments but possible for other feature types as well. Note other attributes such as gene symbols, GeneIDs, and transcript or protein accessions may occur on multiple features, whereas the ID is globally unique for an individual file.

Parent
ID of the parent of the feature

Dbxref
A set of comma-separated tag:ID pairs corresponding to the /db_xref qualifiers provided in the source annotation. Note that database IDs can contain colons, so a format such as “HGNC:HGNC:1100” is expected and should be parsed on the first colon. See NCBI’s documentation on the db_xref qualifier for more details, including URLs corresponding to specific database tags.

Most Dbxref tags known to NCBI are also available in the list provided by the Gene Ontology Consortium .

Name
A suggested display name for the feature, currently populated for specific features: region “landmark” feature – chromosome or linkage group, if available gene – gene symbol or locus_tag RNA (multiple types) and CDS – product accession.version (if exists)

Note
feature comment. This appears as a /note qualifier in the GenBank format. Additional text may appear in the flatfile /note that internally is not part of a comment, and is not included in the GFF3 Note attribute.

Is_circular
provided on the landmark “region” feature as ‘Is_circular=true’ if the record is annotated to be a circular chromosome.

Unofficial attributes

This section describes the unofficial attributes that are the most informative for interpreting NCBI’s GFF3 files. Many of the unofficial attributes correspond to qualifiers that are described for the [GenBank flatfile format](http://www.insdc.org/files/feature_table.html#7.2](http://www.insdc.org/files/feature_table.html#7.2)

anticodon
Position of the anticodon on the seq-id for a tRNA feature

description
The gene full name, corresponding to /gene_desc in the flatfile format.

exception
indicates that the transcription or translation product of the feature as derived from the annotated sequence does not match the sequence of the product. There are several common usages in annotations from the RefSeq eukaryotic genome annotation pipeline (e.g. RefSeq annotation for human):

  1. features corresponding to known RefSeq transcripts (NM_) or proteins (NP_) that have mismatches, indels, or additional sequence compared to the genome. These use the “exception=annotated by transcript or proteomic data”.
  2. features corresponding to model RefSeq transcripts (XM_) or proteins (XP_) that have additional sequence compared to the genome. These use “exception=annotated by transcript or proteomic data”.
  3. features corresponding to model RefSeq transcripts (XM_) or proteins (XP_) annotated as having indels compared to the genome. These use “exception=unclassified transcription discrepancy” or “exception=unclassified translation discrepancy”.
  4. features that undergo programmed genomic rearrangements in order to generate a functional transcript and protein product. These use “exception=rearrangement required for product”.
  5. CDS features that undergo +1 or -1 translation frameshifts (aka ribosomal slippage) use “exception=ribosomal slippage”. See Additional Details for more information.

These and other exceptions may also appear in INSDC annotations, with similar meanings.

exon_number
The GFF3 specification does not include explicit numbering of exons, as is sometimes found in GTF files. Occasionally an exon_number attribute is present on exon features that are annotated separately from an RNA feature. Exon features that are children of mRNA or other transcript features do not include an explicit exon_number attribute, and their relative order should be inferred from the order on the genome.

gbkey
The original GenBank feature type, before conversion into the SOFA type indicated in column 3. INSDC describes feature keys in the DDBJ/ENA/GenBank Feature Table Definition.

gene
The primary gene symbol.

gene_biotype
Attribute computed on gene features based on the set of child features to indicate the overall biotype for the gene annotation at this location. This attribute was introduced in June 2015, and is not present in older files. Values are:

protein_coding: gene has at least one CDS feature, and is not a Ig/TCR segment or pseudogene

V_region, V_segment, D_segment, J_segment, C_region, tRNA, rRNA, snRNA, snoRNA, tmRNA, miscRNA: gene has one or more of the child features, and no other type, and is not a pseudogene

V_region_pseudogene, etc: same as above, but the gene is marked as a pseudogene

transcribed_pseudogene: only has miscRNA child feature(s) marked as pseudogene

lncRNA, and additional ncRNA classes: gene has one or more child ncRNA features of a single ncRNA_class. If ncRNA_class=“other”, then the gene_biotype=ncRNA. INSDC provides a controlled vocabulary for ncRNA classes .

miRNA: a gene with only miRNA child features, or both miRNA and precursor_RNA

segment: gene has a CDS with a “rearrangment required for product” exception, no specific region or segment feature, and is not a pseudogene

segment_pseudogene: same as segment, but is a pseudogene

pseudogene: marked as a pseudogene, but does not meet any of the more specific criteria above.

other: rare cases where gene does not meet any of the above criteria. For example, a partial gene with only mRNA and no CDS child feature(s) is sometimes annotated by RefSeq to represent a location that contains only 5' or 3' UTR, or a gene feature that is not marked as a pseudogene but has no child features.

Note: There may be multiple gene features on a single assembly annotated with the same GeneID dbxref because they are considered to be different parts or alleles of the same gene. In these cases, it’s possible for the gene features to be annotated with different gene_biotype values, such as protein_coding and transcribed_pseudogene or protein_coding and other.

gene_synonym
One or more additional synonyms for gene symbol.

locus_tag
locus_tag is intended to be a unique value on each gene feature, and is also propagated to child features. In some cases it may not be unique because of errors in data files. Annotations from the RefSeq eukaryotic annotation pipeline do not use locus_tag.

ncrna_class
An additional qualifier provided on ncRNA features to indicate a specific feature subtype, such as miRNA or lncRNA. INSDC provides a controlled vocabulary for ncRNA classes .

part
A proposed attribute for the official GFF3 specification that has not yet been finalized. Indicates the order in which multiple rows for the same ID should be joined, using the format “part=X/Y”, where X is the row order and Y is the total number of rows. Only found on some types of features such as genes that are rarely represented as more than one range.

partial
Indicates that the feature is considered to be partial, either internally or at one or both ends. The start_range and end_range attributes indicate which end of an interval is partial. The partial attribute always appears with the value ‘true’ (i.e. partial=true).

product
Name of the transcript or protein product, corresponding to /product in the GenBank flatfile format.

protein_id
Accession.version of the product record for the protein feature, if one exists.

pseudo
The feature, or its parent, is annotated as a non-functional version. If present, it is always provided with the value ‘true’ (i.e. pseudo=true).

pseudogene
A more specific attribute indicating the feature is considered to be a pseudogene. INSDC provides a controlled vocabulary for the pseudogene qualifier .

start_range

end_range

Attributes adapted from GVF to indicate partial feature boundaries. The value is two integers (or a ‘.’ for an unknown value) separated by a comma, where the two values indicate the range of ambiguity for that boundary. The start_range attribute applies to column 4, and end_range applies to column 5. If a ‘.’ is used for the outer range value, as is always the case in NCBI’s current files, then presence of a start_range attribute can simply be interpreted as column 4 is partial, and an end_range attribute as column 5 is partial, regardless of strand, without further analysis of the tag value. Further details about the attributes are available in the GVF specifications provided by the Sequence Ontology.

transcript_id
Accession.version of the product record for the transcript feature, if one exists.

transl_except
One or more translation exceptions, found on CDS features. These indicate codons on the genome (in coordinates corresponding to the column 1 seq-id) that should be considered to be translated as a different amino acid than expected by the codon sequence and translation table for this organism. The format corresponds to that used in the flatfile format, with some encoded characters. For example: transl_except=(pos:25802093..25802095%2Caa:OTHER) means the codon at the indicated position should be translated as “X”, typically instead of a stop codon.

Annotation Data Model

The annotation in GFF3 format reflects the annotation in the source GenBank or RefSeq records, adapted according to the GFF3 specifications. Thus, the data model may vary depending on what information was provided by the annotation submitter. Annotations in RefSeq are more standardized through the use of NCBI’s own annotation pipelines or additional cleanup steps applied when propagating INSDC submitted annotation into RefSeq records.

In general, annotations conform to the central dogma of gene-mRNA-CDS, or gene-RNA for non-coding genes, with some exceptions:

Protein-coding gene annotations in prokaryotes, organelles, and some eukaryote records lack mRNA features in INSDC and RefSeq annotations, and therefore lack mRNA features in the GFF3 files, as allowed by the GFF3 specifications. See “NOTE 2” in the GFF3 specifications for more details.

Gene segment annotations for immunoglobulin and T-cell receptor genes that undergo genomic rearrangements are commonly represented by gene, C/V/D/J_gene_segment, and CDS features corresponding to individual segments of the overall gene. The C/V/D/J_gene_segment feature with one or more child exon features can be treated similar to a transcript feature. Currently, the CDS features for gene segments use the Gene feature as the Parent.

Some pseudogene annotations are represented by a CDS feature with no corresponding mRNA Parent feature.

Some RNA annotations, especially tRNAs, may not have a corresponding Gene parent. This is more common in older annotation submissions.

RefSeq annotations from NCBI’s eukaryotic genome annotation pipeline include additional conventions that may be relevant for processing.

Notable conventions used in RefSeq annotations from NCBI’s eukaryotic genome annotation pipeline are:

Transcribed genes, both protein-coding and non-coding, may be annotated with multiple transcript variants and protein isoforms. A single gene may have both “known” (NM/NR/NP) and “model” (XM/XR/XP) RefSeq products. Known RefSeqs are created both through curation done at NCBI, and automated processes using annotated INSDC sequences, whereas model RefSeqs are created by automated processes primarily based on alignment evidence supplemented with ab initio prediction.

A protein-coding gene may have both coding and non-coding transcript child features. Coding transcripts are represented by mRNA-exon-CDS features, and non-coding transcripts are represented by transcript-exon features.

Non-transcribed pseudogenes are annotated with a gene feature, and may have child exon features if sufficient evidence was available to define the pseudo exon structure. No mRNA or CDS feature is annotated because the gene is thought to not produce an RNA or protein.

Transcribed pseudogenes are annotated with a gene feature and one or more transcript-exon features.

MicroRNAs are annotated as gene-primary_transcript-exon features based on the stem-loop sequence identified by miRBase. Additional ncRNA child features with the attribute ncrna_class=miRNA are annotated to represent the mature miRNA products. The miRNA features currently have the Gene feature as the parent.

Additional non-coding transcripts are annotated as ncRNA, rRNA, or tRNA features with one or more exon child features.

Both “known” (NM/NR/NP) and “model” (XM/XR/XP) RefSeq transcript and protein products may have differences (mismatches or indels) or additional sequence compared to the sequence of the corresponding genome feature. These differences are conveyed in several ways:

A Note describing the differences is provided on the affected transcript or CDS feature.

For RefSeqs with indels or additional 5' or internal sequence, an alignment between the transcript and genome is provided that can be used to map between genome, transcript, and protein coordinates. These alignments are found at the end of each seq-id. See below for details on the alignment format specifications.

More recent GFF files under the /genomes/<genus_species> FTP path incorporate the use of short overlaps or micro-introns to adjust the annotated feature for insertions or deletions in the coding region compared to the RefSeq product. See Additional Details below for more information.

For CDS features with internal stop codons compared to the RefSeq product, a transl_except attribute is provided to indicate the location of the stop codon, and the amino acid that is thought to be encoded at that position. Model RefSeqs use aa:OTHER to indicate the stop codon in the genome may be in error, but the correct sequence is unknown. Known RefSeqs may also use aa:OTHER in older files (fixed in mid 2015). The transl_except attribute is also used to indicate stop codons in the genome that are naturally translated as selenocysteine (Sec).

RefSeqs with additional sequence that does not align to the genome, with the exception of 3' polyA tails and very short unaligned 5' sequence, are annotated with partial features. This markup includes a partial=true attribute on all rows of the affected feature, and a start_range and/or end_range attribute on the specific rows that are marked as partial (see above for a description of start_range/end_range). The most common partial markup is for the 5'-most or 3'-most end of the mRNA and CDS to be marked as partial, but it is also possible for internal sequence to be missing, in which case internal exon and CDS rows will be marked with start_range or end_range.

Additional Details

The source ‘region’ feature

The first feature row for every seqid is a “region” feature spanning the entire range of the sequence, and corresponds to the “source feature” that appears in the GenBank flatfile format. It can be identified by the “gbkey=Src” attribute, and can function as the landmark feature that is used by some software but is not well defined in the current GFF3 specifications. It contains many qualifiers with information about the source of the record. These include:

Name
Currently set to the source chromosome value, if known. The value chosen for Name may be revised in the future to better address requirements for some software.

chromosome
The chromosome of the record, if one is assigned. Note some scaffolds may be assigned to a specific chromosome even if they are not part of a single record for that chromosome (“unlocalized scaffolds”).

Is_circular
see official GFF3 attributes above

genome
The genome source of the record, corresponding to “source genome ” in the ASN.1 specification, and indexed as “gene in [properties]” in the Entrez nucleotide indexing system.

linkage-group
The linkage group of the record, if one is assigned. Similar to chromosome.

mol_type
The molecule type of the record, such as “genomic DNA” or “mRNA”

transl_table
The genetic code table to use for translation of CDS features, if other than the universal table. INSDC provides a controlled vocabulary for genetic code tables .

UTRs

5' and 3' UTR features for mRNAs are not explicitly annotated, but can be inferred from the difference between the child exon and CDS features. For applications that require explicit five_prime_UTR and three_prime_UTR features, NCBI provides a Python script to add UTR features.

Start and stop codons

start_codon and stop_codon features are not explicitly annotated, but can be inferred from the beginning and end of the CDS feature, if that CDS feature is not partial on the end in question. Partialness is represented by start_range and end_range attributes in NCBI’s GFF3 files, using a format adapted from GVF.

More specifically:

  1. the CDS is 5' partial and does not include a start codon if: a) the first CDS row is on the + strand and has a start_range=.,### attribute. b) the first CDS row is on the - strand and has an end_range=###,. attribute.

  2. the CDS is 3' partial and does not include a stop codon if: a) the last CDS row is on the + strand and has an end_range=###,. attribute. b) the last CDS row is on the - strand and has a start_range=.,### attribute.

If those conditions are not met, then you can infer the start_codon and stop_codon position from the first or last 3 bp of the CDS feature.

Origin-spanning features

If an interval (an individual row) spans the origin of a circular sequence, the column 5 coordinate is extended into virtual space. This means that a parent feature may be extended into virtual space, but its child features are not, and some child features may not fall within the span of the parent. For example:

A simple case, with single interval gene and CDS features that both span the origin:

##sequence-region NC_005213.1 1 490885  
NC_005213.1	RefSeq	gene	490883	491764	.	-	.	ID=gene1  
NC_005213.1	RefSeq	CDS	490883	491764	.	-	0	ID=cds0;;Parent=gene1

A complex multi-exon case, where the gene and mRNA features span the origin, but none of the individual exon or CDS rows span the origin. Note the exon and CDS at 959..966(-) that do not fall within the span of the Parent mRNA:

##sequence-region NC_004367.1 1 149696  
NC_004367.1	RefSeq	gene	138637	150662	.	-	.	ID=gene0  
NC_004367.1	RefSeq	mRNA	138637	150662	.	-	.	ID=rna0;Parent=gene0  
NC_004367.1	RefSeq	exon	959	966	.	-	.	ID=id1;Parent=rna0  
NC_004367.1	RefSeq	exon	140247	140485	.	-	.	ID=id2;Parent=rna0  
NC_004367.1	RefSeq	exon	140075	140167	.	-	.	ID=id3;Parent=rna0  
NC_004367.1	RefSeq	exon	139744	139992	.	-	.	ID=id4;Parent=rna0  
NC_004367.1	RefSeq	exon	139572	139661	.	-	.	ID=id5;Parent=rna0  
NC_004367.1	RefSeq	exon	139294	139458	.	-	.	ID=id6;Parent=rna0  
NC_004367.1	RefSeq	exon	139106	139219	.	-	.	ID=id7;Parent=rna0  
NC_004367.1	RefSeq	exon	138904	139005	.	-	.	ID=id8;Parent=rna0  
NC_004367.1	RefSeq	exon	138637	138818	.	-	.	ID=id9;Parent=rna0  
NC_004367.1	RefSeq	CDS	959	966	.	-	0	ID=cds0;Parent=rna0  
NC_004367.1	RefSeq	CDS	140247	140485	.	-	1	ID=cds0;Parent=rna0  
NC_004367.1	RefSeq	CDS	140075	140167	.	-	2	ID=cds0;Parent=rna0  
NC_004367.1	RefSeq	CDS	139744	139992	.	-	2	ID=cds0;Parent=rna0  
NC_004367.1	RefSeq	CDS	139572	139661	.	-	2	ID=cds0;Parent=rna0  
NC_004367.1	RefSeq	CDS	139294	139458	.	-	2	ID=cds0;Parent=rna0  
NC_004367.1	RefSeq	CDS	139106	139219	.	-	2	ID=cds0;Parent=rna0  
NC_004367.1	RefSeq	CDS	138904	139005	.	-	2	ID=cds0;Parent=rna0  
NC_004367.1	RefSeq	CDS	138637	138818	.	-	2	ID=cds0;Parent=rna0

Annotation accommodations for putative assembly errors

The INSDC annotation specification includes several options for annotating mRNA and CDS features affected by putative assembly errors, and these conventions can also appear in the GFF3 format for both GenBank and RefSeq genomes.

CDSes with internal stop codons that are believed to be in error can be annotated with a transl_except attribute on the CDS feature to indicate the location of the internal stop codon and its replacement amino acid.

mRNA and CDS features disrupted by frameshifting insertions or deletions that are thought to represent assembly errors may be adjusted through the use of short overlaps or “micro-introns” between exon and CDS rows of the same mRNA feature. These adjustments artificially appear to split a single exon into two, but serve to restore the reading frame and allow representation of a complete CDS in the correct frame. In the case of an overlap, the overlapping base does not necessarily represent the correct sequence, but serves to represent an insertion of a length that restores the proper reading frame.

The overlap/micro-intron format is a compromise designed to allow processing by most software; however, some software may not properly accommodate overlaps, in which case the annotation may require modification.

If the software supports internal phase shifts with a partial codon in the middle of the CDS, then an overlap can be adjusted by trimming the coordinate of the first row (modify column 5 if the feature is on the + strand, and column 4 if the feature is on the - strand), without altering the phase column:

original: seq1 GenBank CDS 100 150 . + 0 ID=cds1 seq1 GenBank CDS 150 200 . + 0 ID=cds1

altered: seq1 GenBank CDS 100 149 . + 0 ID=cds1 seq1 GenBank CDS 150 200 . + 0 ID=cds1

If the software does not support either overlaps or internal phase shifts, then the alternative is to adjust the first row by a multiple of three, creating a micro-intron that maintains the proper phase:

original: seq1 GenBank CDS 100 150 . + 0 ID=cds1 seq1 GenBank CDS 150 200 . + 0 ID=cds1

altered: seq1 GenBank CDS 100 147 . + 0 ID=cds1 seq1 GenBank CDS 150 200 . + 0 ID=cds1

Ribosomal slippage

Some proteins are expressed by a process involving +1 or -1 translation frameshifts (aka ribosomal slippage). In this case the CDS feature for what is a single exon is split into two intervals, with either a small gap (positive slippage) or a slight overlap (negative slippage), similar to how some putative assembly error cases are annotated. However, for ribosomal slippage annotations the exons of the corresponding mRNA feature do NOT include the slippage because only the translated protein is affected by the slippage event. These types of CDS features can be identified by the attribute “exception=ribosomal slippage”.

Alignments

Alignments are provided according to the GFF3 specifications. They are identified by the SO type in column 3: cDNA_match – used for mRNA-genome alignments EST_match – used for EST-genome alignments protein_match – used for protein-genome alignments match – used for all other alignments, including transcripts other than mRNA

They appear in several places:

Genomic annotation GFF3 files include alignments for genomic RefSeq (NGs), and those transcript RefSeqs (NM/NR and XM/XR accession prefixes) where the transcript does not map perfectly to the feature (see ANNOTATION DATA MODEL).

Genome-genome alignment files as used by NCBI’s Remap service.

Primary-ALT_LOCI and primary-PATCH alignment files , provided as part of the human and mouse GRC assemblies, indicating correspondence between locations on the primary assembly and sequences of alternate loci.

Transcript alignment files for eukaryotes produced by NCBI’s eukaryotic genome annotation pipeline, provided in the Evidence_alignments sub-directory.

RefSeq transcript and RefSeqGene genomic alignments for human and mouse .

Official GFF3 attributes

ID
A unique identifier for the alignment. Some alignments use a long UUID string as a stable alignment identifier. Multiple rows with the same ID indicate parts of the same alignment. For example, transcript alignments are represented with a separate row representing each exon of the transcript.

Target
The accession.version of the sequence aligned to the reference sequence indicated in column 1. The format is “target_id start end strand”, where start < end.

Gap
Location of Gaps (indels) in the alignment in the Exonerate CIGAR format where ‘M’ indicates an aligned base (which may be either a match or mismatch). The locations of mismatches must be computed by comparing the Target and Reference sequences. The Gap attribute is omitted if there are no indels within the alignment Target range.

The Gap attribute is relative to the Target sequence and range for that row of the alignment. If the Target is in minus orientation, then the Gap string is read from end to start. For example: ID=aln1;Target=NG_033055.1 1 7866 -;Gap=M7047 D1 M819 indicates: NG_0033055.1:7866..820 “match” followed by a deletion of 1 bp (the extra base is present in the
reference sequence, and missing from the target) NG_0033055.1:819..1 “match”

Note gaps can also be represented by ranges of the Target (either at the beginning, end, or internally) that are not found in any row of the alignment.

Unofficial attributes

A wide variety of scores may be present in alignment files depending on the process that generated them. Many aren’t of direct use to most users. Most reported scores, including all scores listed below, apply to the alignment as a whole (all rows with the same ID). The most useful scores are described here:

gap_count
number of gap openings in the alignment. Equal to the number of D or I codes in the Gap string.

num_ident
number of identities in the alignment

num_mismatch
number of mismatches in the alignment. If num_mismatch=0, then there are no mismatches within the aligned portion of the Target and Reference (for any match row with the same ID)

pct_coverage
Gapped percent coverage of the Target sequence.

pct_identity_gap
Gapped percent identity, for the aligned portion of the Target. This is the standard BLAST percent identity score. Unaligned portions of the Target (not included in any match row with the same ID) do not affect the reported identity.

pct_identity_gapopen_only
Percent identity, counting gaps as a mismatch regardless of length.

pct_identity_ungap
Percent identity, ignoring gaps

reciprocity
Used for assembly-assembly alignments. Values: 3 == alignment is best for both Reference and Target. Also referred to as ‘First-Pass Alignments’ 1 or 2 == alignment is best for one sequence, but a better alignment is reported for the other sequence. Also referred to as ‘Second-Pass Alignments’

More details on some of the reported scores is available in the NCBI C++ Toolkit documentation

Generated October 22, 2021