The phase III MONALEESA-2 study (clinicaltrials.gov, NCT01958021) reported that addition of the cyclin-dependent kinase (CDK)4/6 inhibitor ribociclib to letrozole is well tolerated and significantly improves progression-free survival (PFS) compared with letrozole alone as a first-line therapy for HR+, HER2− advanced breast cancer [13]. Here, CDK4 is linked to breast cancer.