BCAS3 (breast carcinoma amplified sequence 3), also designated MAAB or GAOB1, is a 913 amino acid protein that is believed to be involved in breast cancer progression. The gene is regulated by ERå (estrogen receptor alpha) and expressed in multiple tissues, including malignant human brain lesions. It is overexpressed and amplified in breast cancer cell lines. BCAS3 contains three WD40 repeat regions, a bromodomain, a rare zinc-finger motif, four probable DNA-binding domains, and two kinase-inducible phosphorylation domains. Five variants are produced due to alternative splicing. BCAS3 interacts with histone H3 and PCAF, which is indicative of histone acetyltransferase activity. BCAS3 also exhibits ERå transactiviation activity by acting as a coactivator with PELP1 or MTA1. The amplification and translocation between the BCAS3 gene and the BCAS4 gene results in a fusion transcript is overexpressed in MCF-7 cells.