In the patients with IPEX syndrome, the lack of the forkhead box protein 3 in lymphoid tissues (thymus, spleen, and lymph nodes), encoded by FOXP3 gene at Xp11.23-Xq13.3, leads to impaired development and suppressed function of CD4+CD25+ T-reg. Here, FOXP3 is linked to immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome.