Radiolabeled large (e.g., antibodies or their fragments, minibodies, and affibodies) and small (e.g., peptides and non-peptides) molecules have recently emerged as promising radiotracers for the imaging PD-1 or PD-L1 levels in cancer patients because they may guide therapy decisions and help in monitoring patients under immunotherapies [288]. The gene discussed is PDCD1; the disease is cancer.