PB is already approved for front-line treatment of HR-positive and HER2-negative breast cancers in combination with endocrine therapy.30 A phase I clinical trial of the structurally similar CDK4/6 inhibitor ribociclib has shown promise in pediatric solid tumors,59,60 suggesting that CDK4/6 treatment is tolerable. The gene discussed is ERBB2; the disease is breast carcinoma.