PADI4 and rheumatoid arthritis: Even though several genes have been identified as genetic factors that contribute to RA susceptibility, such as HLA-DR, PTPTN22, CTLA-4, and PADI4, we still do not have a clear picture of how these genetic factors induce the appearance of RA [4]; it rather seems to be a polygenic disease, where a combination of gene mutations might be needed for disease presentation.