PDCD1 and melanoma: Mechanisms that lack sufficient clinical evidence but are at the forefront of research include alterations in metabolism, epigenetic regulation, and gut microbiota activation.140 Response to anti-PD-1 and clinical benefit has been found in nine PD-1-refractory melanoma patients when combining faecal microbiota transplant before anti-PD-1 therapy.141,142 As the most effective ICB drugs at present, we take anti-PD-1/ anti-PD-L1 drug resistance as the starting point to discuss the specific mechanism of immune resistance.