To address our current limited knowledge of the treatment of autoimmunity in T1D, we have initiated a mechanistic clinical study programme that combines novel study design with experimental medicine to understand how interleukin-2 (IL-2, aldesleukin, Proleukin®), a critical cytokine mediating immune activation and tolerance (unresponsiveness to self) in humans, modulates the immune system in T1D [15]. This evidence concerns the gene IL2 and type 1 diabetes mellitus.