Target heterogeneity was also explored in the ZEPHIR trial, which utilised 89Zr-trastuzumab PET to assess intra-/inter-patient heterogeneity in HER2 expression in the metastatic setting.51 Of the 56 patients assessed, 46% showed 89Zr-trastuzumab avidity in less than half of their FDG-PET/CT tumour volume and experienced shorter time to treatment failure. The gene discussed is ERBB2; the disease is neoplasm.