PTPN22 and systemic lupus erythematosus: Our reasons for addressing B cells as a particular cell of interest in SLE were: 1) SLE is associated with autoantibodies, which are made by B cells; 2) there are a number of susceptibility loci for SLE in genes with important roles in B cell signaling and function, such as LYN, BLK, BANK1, PTPN22, TNFAIP3, and TNIP129; and 3) the only targeted therapy licensed for SLE targets B cells specifically.