PDCD1 and neoplasm: Novel checkpoints like lymphocyte activation gene-3 (LAG-3), T cell immunoreceptor with immunoglobulin and immunoreceptor tyrosine-based inhibitory motif domains (TIGIT), and T cell immunoglobulin and mucin domain-containing protein 3 (TIM-3) also play non-redundant roles in immunosuppression; while monotherapies against them have limited efficacy, combinations with PD-1 blockade show enhanced anti-tumor responses (142–151).