In 2011, a Swedish study found that more than one copy of glutathione S-transferase M1 (GSTM1), a member of the GST family, seems to be a risk factor for autoantibody-positive RA in non-smoking females of age older than 60 years, and GSTM1 acts as a protective factor in ACPA-negative smoking men, suggesting that the copy number of the GSTM1 gene is correlated with the development and severity of RA (Lundström et al., 2011). Here, PRTN3 is linked to rheumatoid arthritis.