Among patients receiving either drug, thrombocytopenia is a frequently observed adverse event, as documented in the pivotal trials [1–5] that led to these drugs’ approval as treatments for metastatic HER2-positive breast cancer, metastatic HER2-low breast cancer [1, 4], and residual invasive HER2-positive breast cancer [3]. The gene discussed is ERBB2; the disease is breast carcinoma.