MMP13 and rheumatoid arthritis: We examined the expression of cytokines interleukin 6 (IL-6), IL-23A, C-X-C motif ligand 1 (CXCL1), CXCL8, matrix metallopeptidase 1 (MMP1) and MMP13, which exacerbate the aggressive phenotype of RA FLSs.