The gut is the major H2S-producing organ, where the gut microbiota is responsible for about half of this production [157] and the inhibitory effect of methionine restriction could be replicated using FMT from methionine restricted mice and ameliorated by the supplementation of a hydrogen sulfide donor, GYY4137, which could rescue MR diet-induced resistance to anti-PD1 immune therapy in tumour bearing mice. The gene discussed is PDCD1; the disease is neoplasm.