CDK4 and breast cancer: The results revealed that protein expression levels of KIFC2, USP9X, and CDK4 were elevated in HR+/HER2– BC tissues relative to matched normal controls (Figure 7, A and B) and that there was a positive correlation in protein levels between KIFC2 and CDK4 (Figure 7C) as well as USP9X and CDK4 in these samples (Figure 7D).