The S100 proteins are released into serum at highly elevated levels in the most severe form of JIA, systemic onset JIA (sJIA), where they correlate well with disease activity—as assessed by physicians’ global assessment of disease activity (r = 0.62), Childhood Health Assessment Questionnaire (r = 0.56), and active joint count (r = 0.46)—and with CRP (r = 0.71) and ESR (r = 0.72) (for all p < 0.001) [20]. Here, CRP is linked to systemic-onset juvenile idiopathic arthritis.