Polycomb group proteins are implicated in embryogenesis and carcinogenesis through transcriptional regulation of target genes. ASXL1 and ASXL2 genes, encoding Polycomb group protein with ASXN and ASXM domains, are human homologs of Drosophila additional sex combs (asx) gene. Exons 2-13 of the ASXL2 gene are fused to exons 1-14 of the MYST3 gene in a case of therapy-related myelodysplastic syndrome due to t(2;8)(p23.3;p11.2). Here, we identified the ASXL3 gene, a novel human homolog of Drosophila asx, by using bioinformatics. ASXL3 gene, consisting of 12 exons, was located within human genome sequences RP11-562H1 (AC023192.8), RP11-265C19 (AC090989.8), and RP11-470B24 (AC010798.9). Complete coding sequence of human ASXL3 cDNA was determined by assembling EST BE145544, exons 4-11, and 5'-truncated KIAA1713 cDNA (AB051500.2). Partial coding sequence of mouse Asxl3 cDNA was derived from 3'-truncated C230079D11 cDNA (AK082659.1). Human ASXL3 mRNA was expressed in pancreatic islet, testis as well as in neuroblastoma, head and neck tumor. Human ASXL3 protein (2248 aa) with ASXN, ASXM and PHD domains was the third member of the human ASXL family. The region between ASXM and PHD domains was divergent among ASXL family members. Proline-rich domain was located within the divergent region of ASXL3, but not within that of ASXL1 and ASXL2. ASXL3-DTNA locus at chromosome 18q12.1 and ASXL2-DTNB locus at 2p23.3 were paralogous regions within the human genome. ASXL3 was a predicted cancer-associated gene, just like ASXL1 and ASXL2. This is the first report on identification and characterization of the ASXL3 gene.