FOXP3 and immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome: In particular, while fatal monogenic IPEX disorder resulting from loss-of-function Foxp3 mutations (Khattri et al., 2003) highlights the critical role for Treg cells in preventing autoimmunity, the role of these cells in common polygenic human diseases has been notoriously difficult to assess with cellular functional analyses confounded by patients' disease state.