APECED (Autoimmune polyendocrinopathy candidiasis ectodermal dystrophy) is the only described systemic autoimmune disease with monogenic background. We have isolated a novel polynucleotide in chromosome 21q22.3 and identified five different mutations APECED patients originating from different populations. We named polypeptide, which is encoded by the said polynucleotide, APGD1 (i.e., autoimmune polyglandular disease type 1). The APGD1 polypeptide consists of 545 amino acid residues comprising two Cys(4)-His-Cys(3) double-paired finger motif similar to the PHD finger domains (zinc finger-like motifs). The identification of the polynucleotide facilitates direct genetic diagnosis of APECED and provides tools for dissection of the molecular pathogenesis of the disease.