BCL2 and breast cancer: Because ABT-263 blocks activity of Bcl-2 and Bcl-xL, both of which are expressed in ER+ breast cancer cells, we tested the impact of selective Bcl-2 inhibition versus selective Bcl-xL inhibition on Mcl-1 protein levels using the Bcl-2 selective inhibitor ABT-199 [16], and the Bcl-xL selective inhibitor A1155463 [21].