Subsequently, mutations in the FOXP3 gene, which codes for a DNA-binding factor with homology to Forkhead family proteins, were found to be associated with the IPEX syndrome (5–8), following the discovery of a frameshift mutation in Foxp3 observed in a naturally occurring mutant mouse (9). Here, FOXP3 is linked to immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome.