Several inhibitory immunoreceptors, referred to as “immune checkpoints”, have been identified and studied in cancer, including, amongst others, programmed death ligand 1 (PD-1), cytotoxic T-lymphocyte associated protein 4 (CTLA-4), lymphocyte activation gene 3 protein (LAG3), T cell immunoglobulin and mucin domain-containing protein 3 (TIM3), and T-cell immunoreceptor with immunoglobulin G1 (Ig1) and immunoreceptor tyrosine-based inhibitory motif (ITIM) domains (TIGIT) (116). Here, PDCD1 is linked to cancer.