Of Fox and Frogs: Fox (fork head/winged helix) transcription factors in Xenopus development

Transcription factors of the Fox (fork head box) family have been found in all metazoan organisms. They are characterised by an evolutionary conserved DNA-binding domain of winged helix structure. In the South African clawed frog, Xenopus laevis, more than 30 Fox genes have been found so far. This review summarises our present knowledge regarding the general structure and common features of the fork head box and will then characterise Fox genes that have been described in Xenopus. Special attention was paid to the temporal and spatial expression patterns during early embryonic development. For some of these genes, the molecular mechanisms leading to their regulation after the onset of zygotic transcription are known. We also report on functional aspects including target gene regulation, cell or tissue specification and interference with the cell cycle. Finally, Fox proteins serve as mediators of signalling pathways and they might function as checkpoint molecules for the cross-regulatory interactions of different intracellular signal transduction chains.