To gain deeper insights into the possible regulators of HER2 mRNA levels, we first analyzed HER2 mRNA expression levels according to tumor addiction to the HER2 oncogene and ER expression in 6 HER2-amplified BC cell lines (HER2/CEP17 ratio: BT474, 6.8; ZR75.30, 7.9; MDAMB361, 5.3; SKBR3, 7.1; HCC1954, 6.7, MDAMB453, 5.2). This evidence concerns the gene ESR1 and breast cancer.