Cyclin-dependent kinases 4 and 6 (CDK4/6) are among the primary proteins responsible for controlling cell cycle entry at the G1/S transition (1, 2, 3, 4) and targeting CDK4/6 is now an Food and Drug Administration (FDA)-approved strategy for the subset of breast cancers that express estrogen receptor (ER). The gene discussed is CDK4; the disease is breast carcinoma.