The drug yielded a lackluster 3% response rate in 71 heavily pre-treated estrogen receptor-positive/human EGFR 2 (HER2)-negative breast cancer patients in the VERITAC phase II single-arm trial investigating ARV-471 in combination with Pfizer’s CDK4/6 inhibitor, Ibrance (palbociclib). This evidence concerns the gene ESR1 and breast cancer.