The acquired PET images are validated by the tissue distribution data. Radiotracer uptake was observed in the lung, liver, spleen and bone, which can be attributed to the moderate expression of both EGFR and HER3 and their role in tissue growth44. This is supported by the data from competitive blocking studies where the tissues (listed above, excluding bone) in the blocked cohort of mice had lower probe accumulation at 48 h than the unblocked cohort (Fig. 2E, Supplementary Table S3). Cross-reactivity of the fully human MEHD7945A with murine EGFR and HER3 is expected; murine EGFR is 94% homologous to human EGFR45 whereas murine HER3 is orthologous to the human receptor with 90.8% sequence homology46. Bone accumulation can be a combination of specific EGFR/HER3 targeted delivery with reported expression of both RTKs in skeletal tissues47,48 and/or non-specific deposit of demetalated 89Zr from its chelate DFO. The latter is evidenced by the free 89Zr observed as early as 24 h with 95–97% of the tracer remaining intact at 120 h (Supplementary Fig. S1). The competitive inhibition experiment of 89Zr-MEHD7945A using unmodified MEHD7945A together with non-specific IgG isotopic probe control from our data is evident of specific tumor targeting of 89Zr-MEHD7945A and thereby eliminating the enhanced permeation effect as the underlying mechanism of delivery. Co-localization of the tracer with EGFR/HER3 expression as shown in the autoradiography and IHC studies further proved the probe’s selectivity for the receptors.