Trastuzumab deruxtecan, an antibody-drug conjugate, has shown promising efficacy in both HER2-high and HER2-low disease.<h4>Case description</h4>We report the case of a 69-year-old woman with a 17-year history of breast cancer, initially diagnosed in 2007 with invasive ductal carcinoma and lymph node metastasis. The gene discussed is ERBB2; the disease is breast carcinoma.