CDK4 and breast neoplasm: In advanced HR-positive breast tumour patients, fulvestrant is often used in combination with a cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor, which can significantly prolong progression-free survival compared to fulvestrant alone (9.2 months vs 3.5 months; P = 0.007) [40]; in patients sensitive to endocrine therapy, this combination therapy also brings about longer OS than fulvestrant monotherapy (39.7 months vs 29.7 months; Hazard ratio, 0.92; 95% CI, 0.55–0.94) [41].