Since the primary role of PD-1 ligands in cancer is seemingly immune inhibition in the tumor microenvironment, and since PD-1 is capable of inhibiting the function of lymphocytes only when engaged with PD-L1 and PD-L2, it is important to understand the pattern of expression of PD-1 ligands in order to determine whether they would be suitable in therapy if blocked. The gene discussed is PDCD1; the disease is cancer.