PDCD1 and cancer: Several immune checkpoints have been discovered and investigated in cancer research in recent decades, including programmed cell death protein 1 (PD-1), lymphocyte-activation gene 3 , cytotoxic T-lymphocyte-associated antigen 4, T cell immunoglobulin and mucin-domain containing-3 , B- and T-lymphocyte attenuator , and the T cell immunoglobulin and immunoreceptor tyrosine-based inhibitory motif domain [51].