ESR1 and breast cancer: Recently, in more refined studies employing immunohistochemical approaches and based on the characteristic expression patterns of ESR1, PR and EGFR (ERBB1/HER1, ERBB2/HER2, ERBB3 and ERBB4), human-like breast cancer phenotypes for CMTs have been developed and classified as luminal A, luminal B, HER2 positive and triple negative (basal-like) [17,18].