To test the effect of genotypes on serum concentrations of YKL-40 in the RA group (age 65 and below) and control group, a multiple linear regression model including serum YKL-40 as dependent variable and a non-linear function of age, case-control status, genotypes and gender as well as the interaction between case-control status and genotype with age as independent variables was applied (Table 2). The gene discussed is CHI3L1; the disease is rheumatoid arthritis.