Trastuzumab (Trz) is a recombinant humanized monoclonal antibody, which effectively reduces the mortality in various malignancies including patients with breast cancer, gastric cancer, and colorectal cancer by inhibiting the human epidermal growth factor receptor 2 (HER2) (Alsina et al. 2022; Vega Cano et al. 2022; Bando et al. 2023). The gene discussed is ERBB2; the disease is breast cancer.