The PTPN22 gene, encoding the tyrosine phosphatase LYP and shown to act as an inhibitor of T-cell receptor signaling, activation and proliferation, has a common missense variant Arg620Trp (rs2476601C>T), the minor allele of which (Trp620) predisposes to several autoimmune diseases, including SLE and T1D (16). The gene discussed is PTPN22; the disease is systemic lupus erythematosus.