CTLA4 and Autoimmunity: As one of the most important T cell populations in maintaining immunological self-tolerance and homeostasis, Treg cells play indispensable roles in autoimmunity.821,822 Mutations in Foxp3 gene cause immunodysregulation polyendocrinopathy enteropathy X-linked syndrome (IPEX) which is a rare chromosome X-linked immunodeficiency syndrome with severe autoimmune disorders.823,824 Furthermore, mutations of Treg cell-related signature genes, such as CD25,825 CTLA-4,826,827 LRBA,828 and AIRE,829,830 result in Treg cell abnormality and severe autoimmune disorders.