MMP9 and rheumatoid arthritis: Known genetic risk factors for RA include anticitrullinated protein antibodies (ACPAs), specific class II human leukocyte antigen (HLA as a major histocompatibility complex), Dickkopf-related protein (DKK1), and matrix metallopeptidase 9 (MMP-9), whereas known nongenetic risk factors include smoking, microbiota, and female sex [14–18].