CTLA4 and neoplasm: In 2015, two studies published in Science unraveled this association between the gastrointestinal microbiota and ICI efficacy in preclinical mouse models, showing that CTLA-4 and PD-1 blockade only reduced tumor growth in mice that carry within their commensal intestinal flora Bacteroides and Bifidobacterium species, respectively.13,14 Thus, tumors in broad-spectrum antibiotic (ATB) treated mice or in germ-free (GF) mice that lacked Bacteroides species were resistant to the CTLA-4 blockade.