Mutations or SNP (single nucleotide polymorphism) of FOXP3 gene may modify its role functionally or quantitatively, therefore leading to the absence of functional CD4+CD25+ Tregs, resulting in some autoimmune diseases (Wildin et al. 2002), such as immunodysregulation, polyendocrinopathy, enteropathy, X-linked IPEX (immunodysregulation polyendocrinopathy enteropathy) syndrome (Vliet and Nieuwenhuis 2007), type 1 diabetes (T1D) (Bassuny et al. 2003), and autoimmune thyroid diseases (Ban et al. 2007). The gene discussed is FOXP3; the disease is type 1 diabetes mellitus.