The main findings for this study were: (1) a technically robust and specific assay towards the CILP-M neo-epitope was developed; (2) CILP-M was primarily generated by MMP-1, MMP-8 and MMP-12, confirmed by the CILP-M ELISA assay and Western blot; (3) CILP-M was measurable in human serum, and the levels of CILP-M were increased in patients with RA, AS and OA as shown in the discovery and validation cohort; (4) CILP-M was also able to discriminate between patients with RA, AS or OA and healthy donors with AUC > 0.90–0.95) CILP-M levels decreased after anti-TNF-α therapy in patients with AS. The gene discussed is MMP8; the disease is rheumatoid arthritis.