ESR1 and breast cancer: Based on these molecular markers, breast cancer can be classified into four basic subgroups, i.e., [ER+|PR+]HER2− (positive ER and PR status, and negative HER2 status), [ER+|PR+]HER2+ (positive ER, PR and HER2 status), [ER−|PR−]HER2+ (negative ER and PR status, and positive HER2 status), [ER−|PR−]HER2− (negative ER, PR and HER2 status)4.