PDCD1 and melanoma: Frankel et al. investigated the effect of the baseline gut microbiome on clinical outcomes in advanced melanoma patients receiving Ipilimumab (anti-CTLA-4), Nivolumab (anti-PD-1), and a combination of Ipilimumab + Nivolumab or Pembrolizumab (anti-PD-1), and reported that an enrichment of Bacteroides caccae was higher in patients who responded to immunotherapy (responders) (n = 24) compared to those who did not (non-responders) (n = 15) [20].