RB1 and neoplasm: Besides, the mechanism of CDK4/6 inhibitor resistance and the identification of sensitive predictive markers have also been reported, including acquired RB1 mutations, loss of RB1, loss of function mutations of FAT-1, CCNE1 overexpression, CDK6 overexpression, CCNE1/RB1 ratio, interferon β expression (55), CDK4 phosphorylation and tumor cloning kinetics (2, 56, 57).