Although Bcl-2/Bcl-xL inhibition using the BH3-mimetic ABT-737, or Bcl-2 specific inhibition, using the BH3-mimetic ABT-199, had little activity as single agents in breast cancers, their combination with tamoxifen resulted in tumor regression in some, but not all, patient-derived ERα+ breast cancer xenografts tested13, supporting a role for Bcl-2 in endocrine resistance. This evidence concerns the gene ESR1 and breast cancer.