Gene Expression Omnibus. GEO is a gene expression data repository and online resource for the retrieval of gene expression data from any organism or artificial source. Many types of gene expression data from platform types, such as spotted microarray, high-density oligonucleotide array, hybridization filter, and serial analysis of gene expression (SAGE) data, are accepted, accessioned, and archived as a public dataset. [See the GEO chapter (Chpater 6) or the GEO web page.]