CDK4 and breast carcinoma: RB1 activity is frequently lost in breast cancer because of phosphorylation by Cyclin D1‐CDK4/6 complex.[13, 35] Our previous study suggested that KLF5 can increase Cyclin D1 transcription.[13, 36, 37] KLF5, XPO1, and RB1 were all expressed in the HCC1806 and SUM149PT cell lines (Figure4A).