CDK4 and neoplasm: Cyclin D is required for estrogen-dependent gene expression, which indicates that cyclin D expression may participate in tumorigenesis through the estrogen receptor signaling pathway, thus promoting tumor growth.6 A preclinical study showed that CDK4/6 inhibitors combined with ET was significantly associated with inhibiting tumor growth.13 The use of CDK4/6 inhibitors (ribociclib, palbociclib, and abemaciclib) is one of the most recent treatments developed for hormone receptor–positive, ERBB2-negative metastatic breast cancer.