FOXL1 is a 337 amino acid protein encoded by the mouse gene Foxl1. FOXL1 belongs to the fork head family and contains one fork head DNA-binding domain. The HNF3/fork head family includes a large number of transcription factors that share a structurally related DNA binding domain. Fork head factors are known to play important roles both during development and in adults. FOXL1 is a winged helix transcriptional regulator expressed in the mesenchymal layer of developing and mature gastrointestinal tract. FOXL1- deficient mice exhibit various defects not only in the epithelial layer of the gastrointestinal tract but also in gut-associated lymphoid tissues. In the small intestine of FOXL1-deficient mice, the formation of Peyer