CHI3L1 and rheumatoid arthritis: Autoantibodies to YKL-39 were detected in 8–11.8% (Sekine et al. 2001; Tsuruha et al. 2002) of patients with RA, and in 11.1% (Tsuruha et al. 2002) of patients with OA, while only 1% of patients with RA had autoantibodies to YKL-40 (Sekine et al. 2001).