Modica et al. investigated the impact of the gut microbiome on trastuzumab antitumor efficacy in mice models of HER2-positive BC and found that the antitumor activity of trastuzumab decreased after fecal microbiota transplantation from antibiotic-treated donor mice10, suggesting a direct involvement of gut microbiome in the efficacy of trastuzumab as well. The gene discussed is ERBB2; the disease is breast cancer.