PDCD1 and cancer: Moreover, MTHFD2 was discovered to be positively associated with several immune checkpoints, such as cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), PD-L1, T-cell immunoglobulin and mucin domain-3 (TIM-3), and Programmed Cell Death 1 (PD-1) in most cancers, especially in BLCA (Figures 1B–E).