The most common CDK4/6i toxicities of any grade observed in pivotal phase III trials were neutropenia, leukopenia, fatigue and nausea for palbociclib (29, 30), neutropenia, nausea, infections, fatigue and diarrhea for ribociclib (31, 32), creatinine increase, diarrhea, fatigue, and neutropenia for abemaciclib (33, 34). The gene discussed is CDK4; the disease is neutropenia.