Thus, diabetes heredity is associated with an increased susceptibility to the environment in terms of becoming ‘‘obese’’ in the adipose tissue even when they have small amounts of body fat. If this inappropriate expansion of adipose cell size is related to a reduced stem cell commitment and/or subsequent differentiation of committed preadipocytes is unknown but a current focus of study. Glutamic acid decarboxylase 65, a neuroendocrine enzyme, is a key autoantigen in type 1 diabetes, in Latent Autoimmune Diabetes of Adults and in various neurological diseases. Serum autoantibodies to GAD65 are an important marker in the early prediction and diagnosis of T1D. The closely related 67 kDa isoform, GAD67, is 71% identical in its amino acid sequence but is rarely an autoantigen in T1D, interacts differently with the pyridoxal-59-phosphate co-factor, and has different kinetics for GABA synthesis in enzyme activity assays. Recently, the crystal structures of human GAD65 and GAD67 were determined, and provided a unique insight into the structural basis for autoantigenicity of these closely related isoforms. Analysis of the structures of the protein isoforms has allowed the Cepharanthine identification of independent B-cell epitope clusters that locate on opposing faces of the C-terminal domains on GAD65 but not on GAD67. Structural comparisons revealed two key differences between the isoforms. First, GAD65 is more flexible than GAD67, mainly at the C- terminal domains and at the catalytic loop residues. Second, there are striking differences between these isoforms in their electrostatic charge distribution. These structural and physicochemical differences correlate with known epitope regions in the antigenic isoform GAD65, revealing how the immunodominant epitopes on GAD65 are highly mobile and charged,Catharanthine relative to the corresponding regions in the non-antigenic isoform GAD67. Although anti-GAD67 antibodies are rare, these antibodies may represent a cross-reactive population of anti- GAD65, but this has not been formally tested. We wondered whether this cross-reactivity might reveal insights into the structural similarities between the isoforms. We therefore set out to more closely examine the reactivity of anti-GAD65 and anti-GAD67 in sera selected to contain anti-GAD65. In this study we have shown that anti-GAD67 occur in a minority of patients with T1D or LADA who have anti-GAD65. The autoantibodies were readily detected by RIP, but not by immunoblotting indicating that, like anti-GAD65, they reacted with conformational epitopes. Overall anti-GAD67 were rarely detected in sera that did not contain high levels of anti-GAD65, and levels of anti-GAD67 were generally 1–10% of the level of anti-GAD65 in the same serum, but the affinity of binding was similarly high for each population of antibodies. As expected, anti- GAD67 reactivity was readily inhibited by the addition of unlabelled GAD67, but unexpectedly, it was also strongly inhibited by the addition of unlabelled GAD65, whereas anti- GAD65 reactivity was inhibited only with GAD65. Taken together, these results indicate that for most sera anti-GAD67 represents a minor population of anti-GAD65 reactive with an epitope shared between GAD65 and GAD67. The location of such a cross-reactive epitope is unknown, but several lines of evidence suggest that it is not in the C-terminal domain of GAD65 that have previously been identified as the likely location for major epitopes for anti-GAD65 in diabetes.