The tumor-to-blood ratio of HER2 low-expressing MCF7 xenografts was 1.2 ± 0.2 at 48 h after injection of 30 μg [111In]In-DTPA-trastuzumab-IRDye800CW, which was significantly lower than in BT474 tumors (13 ± 2, p = 0.007). The gene discussed is ERBB2; the disease is neoplasm.