The GeneChip Porcine Genome Array contains 23,937 probe sets to interrogate 23,256 transcripts in pig, which represents 20,201 genes. The domestic pig, Sus scrofa, is an important model organism for biomedical research of human disease conditions, as well as for agriculture research due to its worldwide importance as a food source. Gene sequences for the array were selected from public databases and include content from UniGene Build 28 (August2004) and GenBank mRNA up to August 24, 2004. The Porcine Genome Array was created within the Affymetrix GeneChip Consortia Program and provides scientists with a single array which can be used for studying gene expression in the porcine animal model.