CDKN1B and breast carcinoma: Because Src-dependent dual Y phosphorylation of p27 is associated with reduced p27 protein levels in breast cancer cell lines7, and because Cdk2-dependent phosphorylation of T187 is associated with p27 ubiquitination and degradation10, we next asked if dual Y phosphorylation of p27 within Cdk2/cyclin A complexes would further enhance intra-assembly phosphorylation of T187.