CDK4 and breast cancer: Although cyclin D1 and CDK4 are putative MYC target genes, and required for MYC-mediated transformation in keratinocytes[15,16], the proliferative effect of MYC in breast cancer cells appears to be independent of cyclin D1/CDK4 activation as evidenced by the absence of cyclin D1 up-regulation and CDK4 activation upon MYC induction[11].