PDCD1 and melanoma: Pi et al. established an anti-programmed cell death protein 1 (anti-PD-1) drug-resistant B16-F10 (B16-F10-R) preclinical melanoma Pdcd1 transgenic mouse model system to investigate the expression of programmed death-ligand 1 (PD-L1) and lymphocyte invasion into the tumor niche [39].