PTPN22 and rheumatoid arthritis: While many of the genes we identified have been mapped to RA GWAS loci in previous reports and have documented associations with RA, such as PTPN22 (protein tyrosine phosphatase, non-receptor type 22) [36], others are less well studied, such as PAM (Peptidylglycine alpha-amidating monooxygenase), an enzyme that catalyzed the C-terminal amidation of peptides.