ESR1 and breast cancer: We defined four breast cancer subtypes in our study based on the data for ERBB2 copy numbers and ER and PR receptor statuses: ERBB2+, ER/PR- (i.e., ERBB2+, ER-, PR-); ERBB2+, ER/PR+ (i.e., ERBB2+, ER+, PR-; ERBB2+, ER-, PR+; or ERBB2+, ER+, PR+); ERBB2-, ER/PR+ (i.e., ERBB2-, ER+, PR-; ERBB2-, ER-, PR+; or ERBB2-, ER+, PR+); and ERBB2-, ER/PR- (i.e., ERBB2-, ER-, PR-).