PDCD1 and melanoma: Several clinical studies suggest the correlation between members of the Faecalibacterium genus or the Firmicutes phylum with elevated responses to anti-PD-1 [12] or anti-CTLA-4 immunotherapy [50], and higher survival rates of melanoma patients [51], as their prevalence is positively correlated with higher T cell infiltration in the TME and higher counts of CD4+ and CD8+ T cells at the periphery [48].