S100A9 and rheumatoid arthritis: Several members of the S100 protein family in adults are identified as reliable predictors of erosive arthritis. Few calcium‐binding S100 proteins such as serum calgranulin-B (S100A9), calgranulin-A (S100A8), and calgranulin-C (S100A12) were found to be elevated in patients with erosive RA and not in milder RA patients or healthy adults.