IL1RN and Arthritis: Next, we investigated whether differences in the incidence and macroscopic severity of arthritis could be observed between 12-week-old Il1rn−/− and Il1rn−/− mice that additionally lacked S100a9. We observed a slightly increased disease incidence in Il1rn−/−XS100a9−/− as compared to Il1rn−/− mice (Fig. 1A).