With the highly frequent HER2+ BC, CDK4 is amplified, as well as erb-b2 receptor tyrosine kinase 2 (ERBB2, the gene encoding the HER2 receptor), mutations of tumor protein p53 (TP53), PIK3CA, and phosphatase and tensin homolog (PTEN), and cyclin D1. This evidence concerns the gene CDK4 and breast cancer.