ESR1 and breast cancer: A subset of the estrogen receptor (ER)-negative breast cancer subtype presents overexpression of the genes within the ERBB2 amplicon including GRB7 [32]. GRB7 expression not only positively correlates with ERBB2 expression in human breast carcinoma but with ERBB3 expression as well [33], although the latter maps at chromosome 12q13 [34].