We show that IL-17F signals via the IL-17RA and -RC receptor subunits and causes deleterious effects in islets including expression of inflammatory chemokines, suppression of β-cell identity gene expression and insulin secretion and induction of cell death, all of which could contribute to the progressive loss of functional β-cell mass in type 1 diabetes. The gene discussed is IL17F; the disease is type 1 diabetes mellitus.