ERBB2 and breast cancer: Previous reports have postulated that the more favorable prognosis associated with HER2-low BC may be attributable to less aggressive biological profile, including lower Ki-67 proliferation indices [9–11], and reduced histological grade [10,12,13], potentially distinct molecular characteristics underlying HER2-low IHC expression and/or differences in the intrinsic subtypes between HER2-low and HER2 0 tumors [9,22].