FOXP3 appears to be critical for the development and function of Tregs, and the loss of FOXP3 expression leads to the lack of Tregs and hyperactivation of CD4+ T cells, resulting in lethal autoaggressive lymphoproliferation, whereas overexpression of FOXP3 results in severe immunodeficiency [11]–[13]. This evidence concerns the gene FOXP3 and Immunodeficiency.