CDK4 and breast cancer: CDK4 is crucial for maintaining tumorigenic potential in breast cancer, and its absence or inhibition can prevent the development of mammary carcinomas triggered by the ERBB2 oncogene as also observed for the activating binding partner cyclin D1.323,324 These observations highlight the crucial role of cyclin D1/CDK4 complex as a sensor of oncogenic pathways activated by tyrosine kinases receptors.