When administered as monotherapy, TKIs have failed to demonstrate clinical benefit in patients diagnosed with HER2-low breast cancer, but later research, particularly the phase III DESTINYBreast-04 trial, revealed that administration of the an antibody–drug conjugate (ADC), trastuzumab deruxtecan (T-DXd), significantly increased progression-free survival (PFS) and overall survival (OS) in this population, leading to its approval for the treatment of this disease [18,20,23]. The gene discussed is ERBB2; the disease is breast carcinoma.