It is worth noting that in immunocompetent mice fed with the CTRL diet, oral supplementation of GYY or cysteine had a trend to increase tumour incidence/growth, especially after anti-PD-1 treatment (Figs. 6a and 7h and Extended Data Fig. 8g), suggesting that too much H2S production is detrimental to antitumour immunity. This evidence concerns the gene PDCD1 and neoplasm.