Although this dichotomization drives the decision of whether or not to administer trastuzumab, it was observed that HER2-negative breast cancer is characterized by a wide spectrum of HER2 expression levels and that advanced breast cancer patients harboring HER2-low expression can benefit from the administration of novel anti-HER2 antibody–drug conjugates [49,50]. The gene discussed is ERBB2; the disease is breast cancer.