It is well known that several hematopoietic cells produce IL-10 and its importance for maintaining tolerance within the intestinal microbiota come from experimental observations using IL-10- or IL-10R-deficient mice (both these mouse models develop spontaneous colitis) (see below, [130]), and clinical data (IBD patients are characterized by decreased levels of the anti-inflammatory cytokine IL-10). This evidence concerns the gene IL10 and inflammatory bowel disease.