Expanded autoreactive CD4+ T cells secrete inflammatory cytokines such as IFN-γ, IL-2, and IL-17 which may in turn trigger cytotoxic lymphocytes (CTL) to stimulate pancreatic islet inflammation (insulitis) and/or directly kill pancreatic islet β-cells; all these processes together contribute to T1D onset and progression (depicted in Figure 1). This evidence concerns the gene IL17A and type 1 diabetes mellitus.