CDK4 and breast cancer: A recent study also identified several potential tumour genetic factors for CDK4/6i treatment response prediction, including RB1, BRCA1/BRCA2, and CDKN2A/2B/2C mutations and FAT1 loss [5].S1 However, these mutations were found in only a small subset of patients with luminal breast cancer [5], and no definitive factor has been identified to govern therapeutic decisions regarding endocrine plus CDK4/6i treatment.