Ptpn22 is rarely reported in B cells but acts as a negative regulator of Src and Syk family kinases downstream of the T cell receptor (TCR) (29), and mutations in Ptpn22 are associated with an increased susceptibility to autoimmune diseases including rheumatoid arthritis and systemic lupus erythematosus (30, 31). Here, PTPN22 is linked to systemic lupus erythematosus.