Notably, a significant strong correlation between HER2 mRNA and HER2 protein levels, as evaluated by IHC, was found in ER-negative BCs but not in ER-positive BCs of a consecutive cohort of BC patients treated in our institute (Figure S1b), suggesting that ER-HER2 crosstalk also impacts HER2 mRNA levels in HER2-low BCs. This evidence concerns the gene ESR1 and breast cancer.