Abemaciclib highly selective inhibits the complexes CDK4/ cyclin D1 (IC50 =2 nmol/L) and CDK6/cyclin D1 (IC50 =10 nmol/L), with no activity against other CDK/cyclin complexes or cell-cycle-related kinases within the nanomolar ranges, except for inhibition of CDK9 at IC50 at least five times higher. Besides the cell-cycle dependent activity, abemaciclib is able to boost antitumor immunity by potentiating tumor antigen presentation and selectively suppressing proliferation of regulatory T (Treg) cells at the same time. Consistent with its activity against CDK4 and CDK6, abemaciclib inhibits RB phosphorylation and leads to G1 arrest in RB-proficient cell lines. In vitro, treatment with abemaciclib resulted in increased activation of human T cells and upregulated expression of antigen presentation genes in MCF-7 breast cancer cells.