ESR1 and breast cancer: For current users of MHT, a significant difference in the odds of breast cancer was detected for the ER+/PR+/HER2- versus ER+/PR+/HER2+ subtypes [case-case analysis: ER+/PR+/HER2+ versus ER+/PR+/HER2- (aOR = 0.28, 95%CI: 0.09–0.88, p = 0.029)], although the odds ratios were not significantly different across any of the other subtypes: ER+/PR+/HER2- vs. ER-/PR-/HER2+(p(het) = 0.793); ER+/PR+/HER2- vs. ER-/PR-/HER2-+(p(het) = 0.342).