Cytotoxic T-lymphocyte-associated protein 4 (CTLA4) is a protein receptor that downregulates the immune system. CTLA4 gene variants associate with various autoimmune diseases, including type 1 diabetes. Fine mapping of the genetic risk has shown that the genomic region near CTLA4 marked by the single-nucleotide polymorphism (SNP) CT60A/G (rs3087243) acts as a susceptibility factor. Yet, the functional basis for the increased susceptibility conferred by rs3087243 remains unclear. We demonstrate that the length of the dinucleotide (AT)n repeat within the CTLA4 3' untranslated region (3'UTR) strongly associates with the risk of SNP CT60A/G (P<6.5 × 10(-72)). Genomic (AT)n repeat length inversely correlated with CTLA4 messenger RNA (mRNA) and protein levels in islet autoreactive T-cell lines. Transfer of a long (AT)n element into T cells lead to a reduction of mRNA compared to a short (AT)n element. Thus, this study provides evidence for a role of the CTLA4 3'UTR (AT)n repeat in the increased genetic risk for islet autoimmunity associated with the CTLA4 locus.