PTPN22 and systemic lupus erythematosus: Although PEP-deficient mice do not show much phenotype in B cells, mice expressing PEP-R619W carrying the corresponding mutation with human LYP-R620W show hyperactivity of both B and T cells and development of lupus-like disease63,64, suggesting that functional properties of LYP acquired by the R620W mutation cause B-cell hyperactivity.