PTPN22 and autoimmune thyroid disease: Indeed, previous studies by other authors identified genes associated with the development of thyroid autoimmunity including a negative regulator of T-cell activation, CTLA-4 (49), PTPN22 (phosphatase-22) (50), and FOXP3, a key gene regulating the differentiation of T-cells into regulatory cells (Treg) (51).