The results of recent clinical trials indicate that some patients with breast cancer previously categorized as HER2-negative, who have low HER2 expression with IHC 1+ or IHC 2+ and negative FISH results (IHC 2+/FISH−), can benefit from novel anti-HER2 antibody-drug conjugates (ADCs) (Banerji et al., 2019; Modi et al., 2020; Tarantino et al., 2020; van der Lee et al., 2015). This evidence concerns the gene ERBB2 and breast cancer.