aFGF and bFGF display 55 % homology which is limited, however, to a few functional domains. The structure of aFGF, revealed by X-ray crystallography, shows a similar folding structure as IL-1. Because aFGF binds to the same receptor as bFGF, aFGF displays more or less the same spectrum of activities. It is, however, generally less active than bFGF in similar assays. FGF receptors are encoded by a gene family consisting of at least four receptor tyrosine kinases that transduce signals important in a variety of developmental and physiological processes related to cell growth and differentiation.