Fecal material from responders strengthened anti‐tumor T cell responses in GF mice, with increased CD8+ T cells rather than Foxp3+ CD4+ Tregs.[40] Vétizou found that feces from CTL‐associated antigen‐4 (CTLA‐4) antibody (Ab)‐treated metastatic melanoma patients induced the outgrowth of B. fragilis and responses to CTLA‐4 Ab in tumor‐bearing GF mice. Here, CTLA4 is linked to metastatic melanoma.