The following key words were used in the literature search: molecular, imaging, immunotherapy, checkpoint, inhibitor, immune, positron emitting tomography OR PET, single-photon emission computed tomography OR SPECT, programmed cell death protein 1 OR PD-1, programmed death-ligand 1 OR PD-L1, cytotoxic T-lymphocyte-associated protein 4 OR CTLA-4, lymphocyte, tumor, cancer, CD8, CD4, CD3. Here, PDCD1 is linked to cancer.