Stem cells are characterized by self-renewal and multipotency to produce multiple lineages of progenitor and differentiated cells. PROM1 gene encodes CD133 protein, which is a cell surface marker of hematopoietic stem cells, prostatic epithelial stem cells, pancreatic stem cells, leukemic stem cells, liver cancer stem cells, and colorectal cancer stem cells. Here, comparative integromics analyses on PROM1 orthologs were performed. Human PROM1 RefSeq NM_006017.1 was a truncated transcript, while AK027422.1 was the representative human PROM1 cDNA. Chimpanzee PROM1 gene, consisting of 27 exons, was identified within NW_001234057.1 genome sequence. Chimpanzee 5-transmembrane protein CD133 showed 99.2% and 60.9% total-amino-acid identity with human and mouse CD133 orthologs, respectively. Only 2 of 8 Asn-linked glycosylation sites in primate CD133 orthologs were conserved in rodent CD133 orthologs. Comparative proteomics revealed that CD133 orthologs were relatively divergent between primates and rodents. PROM1 mRNA was expressed in human embryonic stem (ES) cells, trachea, small intestine, NT2 cells, diffuse-type gastric cancer, and colorectal cancer. Human PROM1 mRNA transcribed from exon 1A was the major transcript. Comparative genomics revealed that the region around exon 1A corresponding to 5'-UTR of human PROM1 mRNA was not conserved in mouse and rat. Intron 2 of PROM1 orthologs was relatively well conserved among mammals. Tandem TCF/LEF-binding sites with 7-bp spacing within intron 2 were conserved among human, chimpanzee, mouse, and rat PROM1 orthologs. Together these facts indicate that canonical WNT signaling activation is implicated in CD133 expression in ES cells, adult stem cells, and cancer stem cells.