While tucatinib is currently approved for the treatment of HER2+ (IHC 3+ or IHC 2+/ISH+) breast cancer (45, 53), our findings, which are consistent with prior reports (54–59), indicate that few prostate cancers meet HER2+ breast cancer classification criteria and are, instead, HER2-low or HER2-ultralow (Figure 8, A–D). The gene discussed is ERBB2; the disease is breast cancer.