Radiolabeled large (e.g., antibodies, antibody-fragments, minibodies, and affibodies) and small (e.g., peptides, peptidomimetics, and non-peptides) molecules are therefore very attractive as radiotracers for the imaging of PD-1 or PD-L1 up- and dysregulation in cancer patients, because they could support therapy decisions and help to monitor the success of an ongoing therapy. Here, PDCD1 is linked to cancer.