It was shown that the administration of unlabeled trastuzumab 5–60 min before the administration of 89Zr-pertuzumab led to an increase in tumor uptake of 89Zr-pertuzumab compared to mice without pre-administration of unlabeled trastuzumab suggesting treatment with unlabeled trastuzumab may affect the quantification of HER2, despite the fact that both antibodies bind to unique epitopes of the receptor. The gene discussed is ERBB2; the disease is neoplasm.