The Additional sex combs (Asx) gene of Drosophila is an Enhancer of trithorax and Polycomb (ETP), and is required to maintain activation and silencing of homeotic loci. The molecular basis of this dual function is not understood. Here, we identify a human homolog of Asx, termed ADDITIONAL SEX COMBS-LIKE 1 (ASXL1). Overall, the amino acid sequence of ASXL1 has 21% identity and 41% similarity to Drosophila ASX. The ASXL1 protein contains a 118 amino acid conserved amino terminal region of unknown function that we term the ASX homology domain (ASXH) that contains two LXXLL consensus sequences for nuclear receptor binding. ASXL1 also contains a conserved C-terminal cysteine cluster that is a variant of the PHD domain. Three ASXL1 transcripts of differing size are detected, which are widely expressed in adult tissues. ASXL1 maps to chromosome 20q11, a region frequently amplified in human tumors. Interestingly, ASXL1 is overexpressed in cell lines derived from carcinomas.