PDCD1 and colorectal cancer: Responders: an elevation of the Prevotella/Bacteroides ratio; moreover, gut bacteria with the ability of SCFA production, including Eubacterium, Lactobacillus, and Streptococcus, were positively associated with anti-PD-1/PD-L1 response across different GI cancer types (colorectal cancer, n = 19; esophageal cancer, n = 14; gastric cancer, n = 23; Others, n = 18).