ESR1 and breast cancer: A phase I study, the first to evaluate venetoclax for solid tumors, showed promising clinical activity in combination with tamoxifen for estrogen receptor (ER) and BCL-2-positive breast cancer, although a subsequent phase II study showed no benefit from the addition of venetoclax to fulvestrant for patients with ER positive/HER2 negative breast cancer who had progressed on a CDK4/6 inhibitor [131,132].