Another phase 2 trial, NCT02657928 (Colon-Otero et al., 2020), investigated the effects of combined four-week treatment with cyclin-dependent-kinase (CDK) 4 and CDK6 inhibitor Ribociclib (400 mg/day) and Letrozole (2.5 mg/day) in 20 patients with relapsed ER positive endometrial cancer. This evidence concerns the gene ESR1 and endometrial cancer.