With more evidence to good efficacy of trastuzumab deruxtecan (DS-8201a) for HER2-low breast cancer patients, HER2-low is becoming important in some guidelines for molecular classification of breast cancer (up to 55%) (Modi et al., 2020; Li and Jiang., 2021; Diéras et al., 2022; Modi et al., 2022). The gene discussed is ERBB2; the disease is breast cancer.