In CIA, several different cartilage-derived proteins, including type II collagen (CII) (Trentham et al., 1977), type XI collagen (CXI) (Lu et al., 2002a) and cartilage oligomeric matrix protein (COMP) (Carlsén et al., 1998), are used to induce arthritis in rats, although almost all gene-mapping studies (discussed later) have been performed using CII-induced arthritis. The gene discussed is COMP; the disease is arthritic joint disease.