Experimental colitis in IL-10−/− mice is multifactorial but microbiota-dependent,38 as germ-free IL-10−/− mice do not develop colitis39 and colitis can be attenuated or aggravated by colonizing germ-free or specific pathogen-free mice with different bacterial isolates, such as Lactobacillus and Helicobacter species.40 Murine gut microbiota compositions can vary considerably between animals, including mice from the IL-10 deficient lineage that has been continuously bred for multiple generations in our facility (Figure 3b). This evidence concerns the gene IL10 and colitis.