CDK4 and breast cancer: According to the 2022 Japanese Breast Cancer Clinical Practice Guidelines, the standard first-line treatment for postmenopausal HR-positive, HER2-negative metastatic breast cancer includes a non-steroidal aromatase inhibitor combined with a CDK4/6 inhibitor.4) Palbociclib and abemaciclib were approved in Japan in December 2017 and November 2018, respectively.