Similarly, the murine commensal Helicobacter hepaticus (Hh) is found in many academic and commercial mouse colonies12,13 and infection with Hh is linked to chronic hepatitis and hepatocellular carcinoma.14,15Hh can also elicit intestinal inflammation in immunodeficient mice.16,17 Moreover, IL-10- or T cell-deficient mice require Hh for the development of colitis.18–20 However, there are still major gaps in our understanding of the complex interaction between the microbial community, a certain single species or even bacterial antigens, and the host. The gene discussed is IL10; the disease is colitis.