PTPN22 and rheumatoid arthritis: Despite these caveats, the possibility that hypercitrullination of MZB1 is a unique feature of RA-ILD is supported by recent publications showing that several major genetic and environmental risk factors of RA, such as family history, SNPs at the PTPN22 gene, periodontitis, and cigarette smoking, are associated with local or systemic hypercitrullination in healthy individuals (33–36).