In the blood of children with untreated biologic JIA (T0), there was a statistically significant decrease in the concentrations of GAAGs and COMP (p < 0.001), by 51% and 22%, respectively, and a statistically significant increase in the concentration of YKL-40 by 97% (p < 0.001) compared to the concentrations of the evaluated matrix components in the blood of healthy children. The gene discussed is CHI3L1; the disease is juvenile idiopathic arthritis.