The UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (ppGaNTase) family of enzymes initiates mucin-like O-glycosylation of specific proteins. Using exon-prediction analysis on genomic sequence from human chromosome 12p13.3, we identified novel exons that shared significant homology with the ppGaNTases. cDNA library screening and RT-PCR produced the complete coding sequence of a novel human ppGaNTase family member, designated GalNAc-T8. The open reading frame (ORF) of GalNAc-T8 codes for a 637 amino acid, type-II membrane protein that is 45-60% identical to the other mammalian ppGaNTases. GalNAc-T8 shares high homology within the functional regions of the known ppGaNTases; however, the enzyme possesses a novel residue substitution within a characteristic motif of the catalytic domain. Northern analysis of multiple human tissue mRNAs demonstrated that the 5.0 and 2.1kb GalNAc-T8 transcripts are widely expressed. The metabolic disorder autosomal dominant hypophosphatemic rickets (ADHR) was previously mapped to the region of chromosome 12p13.3 in which GalNAc-T8 resides. Using a positional-candidate strategy for identifying the ADHR gene, GalNAc-T8 was subjected to mutational analysis in DNA from ADHR individuals. We detected multiple polymorphisms in the human GalNAc-T8 ORF, but did not find ADHR mutations. In summary, these studies identified the human GalNAc-T8 gene, as well as multiple genomic polymorphisms that will be useful for further understanding the structure-function relations of the ppGaNTases.