Acidic and basic fibroblast growth factor family; FGFs are mitogens, which stimulate growth or differentiation of cells of mesodermal or neuroectodermal origin. The family plays essential roles in patterning and differentiation during vertebrate embryogenesis, and has neurotrophic activities. FGFs have a high affinity for heparan sulfate proteoglycans and require heparan sulfate to activate one of four cell surface FGF receptors. Upon binding to FGF, the receptors dimerize and their intracellular tyrosine kinase domains become active. FGFs have internal pseudo-threefold symmetry (beta-trefoil topology).