CDK4 and Decreased total leukocyte count: While anti-tumorigenic cytotoxic T cells are activated by CDK4/6i29, CDK4/6i resulted in reduced proliferation, survival and differentiation in hematopoietic stem cells30, one reason for the pronounced number of patients with neutropenia, leukopenia, thrombocytopenia and anemia as adverse effect under therapy with CDK4/6i.