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Interleukin ( IL)-21 and protein tyrosine phosphatase non-receptor 22 ( PTPN22) regulate lymphocyte function and have been implicated in the pathogenesis of autoimmune diabetes. We sequenced the proximal promoter of the IL-21 gene for the first time and analysed the PTPN22 1858 T polymorphism in type 1A diabetes ( T1AD) patients and healthy controls ( HC). We correlated the frequencies of islet and extra-pancreatic autoantibodies with genotypes from both loci. The case series comprised 612 T1AD patients and 792 HC. Genotyping of PTPN22 C1858T was performed on 434 T1AD patients and 689 HC. The −448 to +83 base pairs (bp) region of the IL-21 gene was sequenced in 309 Brazilian T1AD and 189 HC subjects. We also evaluated human leucocyte antigen ( HLA) DR3/ DR4 alleles. The frequencies of glutamic acid decarboxylase ( GAD65), tyrosine phosphatase-like protein ( IA)-2, anti-nuclear antibody ( ANA), thyroid peroxidase ( TPO), thyroglobulin ( TG), thyrotrophin receptor autoantibody ( TRAb), anti-smooth muscle ( ASM) and 21-hydroxylase (21- OH) autoantibodies were higher in T1AD patients than in HC. The PTPN22 1858T allele was associated with an increased risk for developing T1AD [odds ratio ( OR) = 1·94; P < 0·001], particularly in patients of European ancestry, and with a higher frequency of GAD65 and TG autoantibodies. HLA- DR3/ DR4 alleles predominated in T1AD patients. A heterozygous allelic IL-21 gene variant (g.-241 T > A) was found in only one patient. In conclusion, only PTPN22 C1858T polymorphism and HLA- DR3 and/or DR4 alleles, but not allelic variants in the 5′-proximal region of the IL-21 gene were associated with T1AD risk. Patients with T1AD had increased frequencies of anti-islet-cell, anti-thyroid, anti-nuclear, anti-smooth muscle and anti-21- OH autoantibodies. The C1858T PTPN22 polymorphism was also associated with a higher frequency of GAD65 and TG autoantibodies. [ABSTRACT FROM AUTHOR]