S100A9 and Arthritis: The cumulative disease activity score of all four feet (range 0–8) correlated well with optical imaging data presented as mean values for all four feet and systemic S100A8/S100A9 levels of 1,180±360 ng ml−1 in mice with mild arthritis (CS2-3) versus 170±30 ng ml−1 in healthy control mice and 2,700±380 ng ml−1 in mice with high disease activity (CS4-6; Fig. 3d,e).