Background: Breast cancer amplified sequence 3 (BCAS3) was identified as a gene overexpressed and amplified in breast cancer cell lines. BCAS3 expression has been shown to be regulated by estrogen receptor alpha (ER alpha) and its coregulator PELP1 (proline-, glutamic acid-, and leucine-rich protein 1). BCAS3 has also been demonstrated to act as a coactivator of ER alpha activity in breast cancer cells and associate with the histone acetyltransferase complex protein P/CAF (p300/CBP-associated factor).