Human FOXP3 Synthetic Peptide. FOXP3 (Forkhead box protein 3) is a member of the forkhead/winged-helix family of transcriptional regulators, highly conserved across mammals, and essential for normal immune homeostasis. FOXP3 is 381 amino acids long, stably and constitutively expressed at a high level in CD25 + CD4 positive regulatory T cells, a low level in CD4-positive/CD25-negative cells, and is absent in CD4-negative/CD8-positive T cells. FOXP3 may be a master regulatory gene, and a more specific marker of regulatory T cells. Defects in the gene encoding FOXP3 protein cause the scurfy phenotype in mice. In humans FOXP3 defects play a role in IPEX syndrome (immune dysfunction, polyendocrinopathy, enteropathy, X-linked syndrome), also know