CDK4 and breast cancer: The main biomarkers involved include phosphatidylinositol-4,5-bisphosphate 3-kinase, catalytic subunit alpha (PIK3CA), estrogen receptor 1 (ESR1), CDK4/6, BRCA1/2, markers for immunotherapy or antibody-drug-conjugates (ADCs), circulating tumor DNA (ctDNA) and circulating tumor cells (CTC), etc. Different genetic testing is recommended for different subtypes of breast cancer, as summarized as follows.