p27 Analogue CP2 is a macrocyclic peptide inhibitor that targets the SCFSkp2/Cks1 E3 ubiquitin ligase complex (Kd=32 nM). By preventing the ubiquitination and subsequent degradation of the p27 protein mediated by SCFSkp2/Cks1, it helps restore p27 levels and inhibits cell proliferation. This compound shows potential for research on cancers that rely on Skp2 overexpression, such as breast cancer.