ESR1 and breast cancer: The hormone receptor based four subtypes of breast cancer considered throughout the study were (i) ER+/PR+/Her2neu+ (triple positive breast cancer, TPBC); (ii) ER-/PR+/Her2neu+ (single negative breast cancer, SNBC); (iii) ER-/PR-/Her2neu+(double negative breast cancer, DNBC) and (iv) ER-/PR-/Her2neu- (triple negative breast cancer, TNBC).