CDK4 and breast cancer: Many investigations have also been devoted to find new therapeutic targets closely associated with breast cancer aggressiveness, invasion, metastasis, and recurrence, such the gastrin-releasing peptide receptors (GRPR) overexpressed on the surface of cancer cells, cyclin-dependent kinase 4 and 6 (CDK4/6), histone deacetylase (HDACs), and MYC proto-oncogene.