A transcriptional regulator primarily expressed in medullary thymic epithelial cells. It plays an important role in immunity by regulating the expression of a wide array of SELF-ANTIGENS and negative selection of autoreactive T-cells in the thymus. Deficiency in this protein induces a rare autosomal-recessive disease called AUTOIMMUNE POLYENDOCRINOPATHY-CANDIDIASIS-ECTODERMAL DYSTROPHY.