PDCD1 and neoplasm: In the current report, we have validated the 64Cu-pembrolizumab tracer in two different mouse models, one bearing a hPD-1-expressing 293 T stable cell line xenograft (NSG/293 T/hPD-1) and the other bearing a human A375 tumor that does not express hPD-1 but in which some infiltrating TILs do express hPD-1 (hNSG/A375).