PROS1 and juvenile idiopathic arthritis: The thesis suggesting the increased enzymatic degradation of ECM fibrous proteins is confirmed by significantly higher concentrations of MMP-1, -2, -3, or -9 demonstrated in the blood, synovial fluid, and saliva of children with JIA, which are not balanced by the concentrations of their tissue inhibitors, i.e., TIMP-1 and -2 [10,25,46,47,48].