CHI3L1 and rheumatoid arthritis: Several independent groups demonstrated that the elevated levels of YKL-40 in serum reflect the degree of the synovial inflammation and joint destruction in patients with RA and OA (Johansen et al. 1999, 2001; Matsumoto and Tsurumoto, 2001; Peltomaa et al. 2001; Conrozier et al. 2000).