CDK4 and breast cancer: To validate the clinical relevance of the KIFC2-USP9X/CDK4 axis in HR+/HER2– BC, we collected another (in addition to those shown in Supplemental Figure 1D) 15 pairs of HR+/HER2– BC specimens and matched adjacent normal samples to determine expression levels of KIFC2, USP9X, and CDK4 by immunoblotting.