Comparative safety and tolerability are a current priority with CDK4/6 inhibitors, and the various toxicities differ in terms of the spectrum and timing of occurrence: neutropenia mainly occurs with palbociclib during the first three cycles of treatment, diarrhea occurs with abemaciclib in 6–8 days, whereas QT prolongation and liver injury mainly occur with ribociclib. The gene discussed is CDK4; the disease is Decreased total neutrophil count.