CDK4 and aneurysmal bone cyst: Early clinical data support activity for CDK4/6 inhibitors in HER2-positive/ER-positive ABC, including a phase I clinical trial published in 2016, which evaluated the safety and the efficacy of abemaciclib, an orally administered CDK4/6 inhibitor, in patients with solid cancer, including 11 patients with HER2-positive ABC.