In mice co-administered a blocking dose (0.5 mg) of unlabelled trastuzumab, a reduction of HER2-mediated [89Zr]Zr-trastuzumab uptake was observed, resulting in intratumoural 89Zr:111In ratios of 0.9 ± 0.1 and 1.4 ± 0.1 for MDA-MB-231 and MDA-MB-231/H2N tumour-bearing mice, respectively. Here, ERBB2 is linked to neoplasm.