There are several large phase III trials of cyclin-dependent kinase (CDK) 4/6 inhibitors in combination with endocrine therapy for ER-positive (ER+) early stage breast cancer.53–55 ER:PR-B interactions may also serve as predictors of benefit to CDK4/6 inhibitors by identifying patients likely to relapse on standard adjuvant endocrine therapy. Here, RB1 is linked to breast cancer.