FGF6 is structurally very similar to the other members of the FGF gene family, and particularly to the FGF4 gene, which was instrumental in its isolation. Its longest open reading frame encodes a 208 amino acid residues long protein, both in man and in the mouse. It is expressed as a 4.8 kb transcript in skeletal muscle. In developing muscle, expression starts at the myotomal stage and culminates in differentiated fetal muscle masses. In culture, FGF6 protein is mitogenic and has a transforming capacity for fibroblasts. It represses the terminal differentiation of myoblasts. Action of FGF6 could be mediated by the FGFR4 receptor, which binds FGF6 and whose gene is also expressed in developing skeletal muscle.