Recently, novel therapeutic agents such as trastuzumab deruxtecan, an antibody–drug conjugate (ADC) of trastuzumab and a ‘tecan,’ have emerged, demonstrating compelling results in particular for the treatment of HER2-low (HER2 1+ or HER2 2+ without in situ hybridization [ISH] amplification) breast cancer [5]. The gene discussed is ERBB2; the disease is breast cancer.