The prognosis for ER+HER2-breast cancer that has recurred early has also improved in recent years owing to CDK4/6 inhibitors.15,16) However, in cases of heart failure, renal failure may also be present, as in this case, where the safety of CDK4/6 inhibitors has not been confirmed, leading to the avoidance of these drugs. Here, ESR1 is linked to breast carcinoma.