The CDK4/6 inhibitors palbociclib, ribociclib, and abemaciclib are each indicated in the first-line for recurrent unresectable or metastatic ER+/HER2-breast cancer in combination with an aromatase inhibitor (AI), with concurrent ovarian function suppression for pre-menopausal women (Gradishar et al., 2022). This evidence concerns the gene ESR1 and breast cancer.