CDKN1B and cancer: Moreover, the capacity for intra-assembly cyclin E/A-CDK2- (but not cyclin D1-CDK4-) mediated p27 phosphorylation to form the T187 phosphodegron (139–141) suggests that in cancer, it is the p27 complexed to cyclin E/A-CDK2—not cyclin D1-CDK4—that is the target for SCF-SKP2-dependent degradation.