Lymphoid phosphatase (LYP) is a protein tyrosine phosphatase (PTP) expressed in hematopoietic cells: A SNP polymorphism (1858C > T) has been linked to a higher risk of developing several autoimmune diseases, including type I diabetes and systemic lupus erythematosus [1]. Here, PTPN22 is linked to systemic lupus erythematosus.