Among the immune-related peptides, D-PPA [NYSKPTDRQYHF] is a PD-L1-binding peptide developed by Chang et al. Their study demonstrated that the hydrophilic D-type polypeptide (D-PPA) itself did not have cytotoxicity; however, through blocking the PD-1/PD-L1 interaction, it could inhibit tumor growth and prolong animal survival in CT26 tumor-bearing mice [18]. This evidence concerns the gene PDCD1 and neoplasm.