Anti-PD-1 antibodies can be labeled with 89Zr or 64Cu and are suitable for in vivo imaging PD-1-expressing tumour-infiltrating lymphocytes [89], which might be an interesting approach for the non-invasive visualisation and quantification of PD-1-expression, as immunohistochemical analyses are limited by the heterogeneous tissue expression on biopsies or single tissue specimen [90]. The gene discussed is PDCD1; the disease is neoplasm.