SKP2 and breast cancer: Expression of PD-L1 in the microarray-based BC TCGA public dataset showed a highly significant negative correlation with p27 but not with p21. It is possible that PD-L1 regulates p21 protein expression at the post-translational level since PI3K/AKT and SKP2 were reported to regulate the protein level of p21 through both transcriptional and non-transcriptional mechanisms [36–38].