PDCD1 and melanoma: Two first-in-human studies in melanoma demonstrated that FMT from prior anti-PD-1 responders can induce microbiome engraftment, reprogram the TME, and convert a subset of anti-PD-1–refractory patients to clinical benefit—providing proof-of-concept that altering microbial ecosystems can modulate antitumor immunity in humans [230, 264].