PDCD1 and metastatic melanoma: In the first study on metastatic melanoma patients in 2017, Frankel et al. found that Bacteroides caccae was enriched in all ICI responders, while Fecalibacterium prausnitzii, Bacteroides thetaiotamicron, and Holdemania filiformis were higher in responders to ICI combination and Dorea formicogenerans was enriched in anti-PD-1 responders [71].