Genetic variations in CD152 have been reported to confer susceptibility to the development of type-1-diabetes mellitus (T1DM). In this protocol we will draw blood samples from patients and normal control volunteers in order to analyze for the presence of CD152 genetic variations and to determine whether those variations are associated with measurable abnormalities in T cell function. Subjects with T1DM and homozygous CD152 gene polymorphisms, patients with T1DM but without CD152 polymorphisms, and control subjects with or without CD152 polymorphisms will be studied to determine whether the CD152 polymorphisms have measurable effects on T cell function and signaling. We will assess T cell function by measuring: proliferation, viability, apoptosis, cytokine production and T cell signal transduction. The identification and characterization of genes that are linked to T1DM may provide an explanation into the mechanisms by which individuals are predisposed to T1DM.