FGF-4, also known as FGFK and K-FGF, is a member of the FGF family of mitogenic peptides. FGF-4 expression is spatially and temporally regulated during embryonic development. FGF-4 plays a key role in limb development and has been identified as the molecular mediator of the activities of the apical ectodermal ridge that is required for directing the outgrowth and patterning of vertebrate limbs.