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. Here, PIK3CA is linked to breast cancer.