Variable expression of clinical arthritis at the level of a single joint. The development of clinical arthritis (mild soft tissues swelling = clinical score 3) post induction day (PID) at the levels of single joints (proximal and distal interphalangeal joints and metacarpal region) is indicated in red. If the swelling was inconclusive, this is indicated with a dashed red line (see right foot and index finger, PID 32). Black arrows point to the variable swelling of the indicated joints. CIA, collagen-induced arthritis.