CDK4 and breast cancer: Palbociclib is an inhibitor of CDK4 and CDK6 that is approved for advanced breast cancer and has been shown to induce cell cycle arrest in CDK4-overexpressing LPS cells.11 In a single-arm phase II study of palbociclib, 48 patients with WD/DDLPS were treated with a dosage of 200 mg once daily for 14 days every 3 weeks.58CDK4 was amplified in 92% of patients, whereas pRb expression was intact in 85% of tumors.