The gene encoding BCAS4 (Breast carcinoma-amplified sequence 4), a 211 amino acid protein, is found in a region on chromosome 20 that is frequently amplified in human breast cancer. This 20q13 locus amplification is found in 12%–39% of primary breast tumors, which correlates with a 17q23 locus amplification that is found in 20% of primary breast tumors. The amplification and translocation between the BCAS4 gene and the BCAS3 gene, with a 17q23 locus, results in a fusion transcript that is overexpressed in MCF-7 cells. Also, deletion of chromosomal region 20q13.13-q13.2 and resultant deletion of BCAS4, as well as three other genes, is the cause of Okihiro syndrome, a disease characterized by ocular and upper limb anomalies. BCAS4 is normally expressed in thymus, kidney, spleen, placenta and brain. There are three isoforms of BCAS4 which are produced as a result of alternative splicing events.