An analysis of fecal microbiota performed on melanoma patients undergoing anti-CTLA-4 treatment revealed differences between subjects prone and subjects resistant to colitis: bacteria within the Bacteroidetes phylum in general, and specifically, the families of Bacteroidaceae, Rikenellaceae, and Barnesiellaceae, were more abundant in patients who did not manifest adverse events. This evidence concerns the gene CTLA4 and colitis.