BCL2 and breast cancer: Tamoxifen upregulated Bcl-2 in PDX models of ER+ breast cancer, whereas ABT-737 and ABT-199 were found to elicit a response in PDX xenografts only in combination with tamoxifen, suggesting the requirement of Bcl-2 upregulation with tamoxifen for the activity of these BH3 mimetics [114].