Many recent studies have revealed that low-dose IL-2 therapy preferentially expands CD4+ regulatory T cell populations in humans; the results from these studies showed that administration of low-dose IL-2 to patients with chronic graft-versus-host disease (GVHD) [10], autoimmune vasculitis induced by hepatitis C virus infection [11], type I diabetes mellitus [12], and systemic lupus erythematosus [13,14] increased the proportions of functional Tregs. This evidence concerns the gene IL2 and chronic graft versus host disease.