SKP2 and breast cancer: Programmed Death-Ligand 1 (PD-L1) promotes tumor progression through several mechanisms, including its intrinsic effect on breast cancer cell proliferation via the S-Phase Kinase-Associated Protein 2 (SKP2)-p21<b><sup>Cip1</sup></b>/p27<b><sup>Kip1</sup></b> (SKP2-p21/p27) axis.