These inhibitors have been used to treat many cancers including colorectal cancer, breast cancer, non-small cell lung cancer, and melanoma (Patnaik et al., 2016), and a phase II study is underway to evaluate effect of letrozole and ribociclib (CDK4/6 inhibitor) in patients with relapsed ER-positive endometrial cancer (NCT02657928). Here, CDK4 is linked to breast cancer.