The Breast carcinoma amplified sequence (BCAS3) gene is localized to 17q23, a region that is often amplified in breast cancers. Copy number gains at this region have also been reported in other tumors such as brain, lung, liver, testis and bladder. BCAS3 is thought to be an estrogen receptor (ER)-alpha co-activator that acts through PELP1, another ER-alpha that in turn activates BCAS3 expression. This may set up a positive feedback loop leading to ER-a signal amplification in the cell which may play a significant role in breast cancer. BCAS3 expression has also been observed in embryonic stem cells and vascular precursors, suggesting that it may also play a role in embryogenesis and tumor angiogenesis.