ESR1 and breast cancer: In stark contrast, patients harboring triple-negative breast cancers (TNBCs), which lack notable expression of ER-α, PR, and HER2, exhibit the highest disease grades and proliferation indices that culminate in (i) the worst progression-free and overall survival rates of all breast cancer subtypes, and (ii) rapid relapse and early death within 5 years of initial diagnosis and treatment [6,7,8].