The rationale for investigating ultra-low dose (ULD) IL-2 treatment in T1D was provided by genome-wide association studies (GWAS) that have identified multiple genes in the IL-2-regulatory T (Treg) cell-T effector (Teff) cell pathways (IL2RA (CD25), PTPN2, IL2, BACH2, CTLA4, IL21, IL6 and IFIH1) associated with the risk of development of T1D [16]. Here, IL21 is linked to type 1 diabetes mellitus.