HR and breast cancer: Based on the expression of hormone receptors (HR) and HER2, BC used to be classified into four intrinsic subtypes: luminal A (HR+ and HER2−), luminal B (HR+ and HER2+), HER2-enriched (HR− and HER2+), and triple-negative BC (TNBC, HR− and HER2−) [33], among which TNBC is generally considered to have the worst prognosis.