PRTN3 and rheumatoid arthritis: Moreover, the occurrence of ACPA-positive RA is related to genetic risk factors that predispose for RA, for instance, protein phosphatase non-receptor type-22 (PTPN22) and MHC class II alleles, and exogenous factors, such as smoking and bacterial infections, which may promote deamination of arginine, through activation of PAD, resulting in the generation of citrullinated antigens and thus in the induction of ACPAs production [70,71].