The concentration of tumor-infiltrating lymphocytes (TILs) expressing immune checkpoints and immune regulatory molecules, such as PD-1, PD-L1, lymphocyte activation gene 3 (LAG-3), T-cell immunoglobulin and mucin domain protein 3 (TIM-3), and indoleamine 2,3-dioxygenase (IDO1), is notably higher in lung metastases (41). The gene discussed is PDCD1; the disease is neoplasm.