CDK4 and breast cancer: For example, increased proliferation by Brn-3b may be associated with its ability to transactivate the promoters of genes required for cell cycle progression such as cyclin-dependent kinase 4 (CDK4) [4] and its regulatory partner cyclin D1 [5], which are required, whilst repressing breast cancer susceptibility gene 1 (BRCA1) [6], which is associated with cell cycle arrest in breast cancer cells.