The aim of this review is to provide information on the role of the Foxp3 protein in the human body and its involvement in the development of two types of primary immunodeficiency diseases: IPEX (Immunodysregulation Polyendocrinopathy Enteropathy X-linked syndrome) and CVID (Common Variable Immunodeficiency). The gene discussed is FOXP3; the disease is immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome.