During post-natal myelination, maximal levels of IGF-I immediately precede periods of active myelination (Carson et al., 1983; Rotwein et al., 1988; Bach et al., 1991) , while the phenotype of transgenic mice that over-express IGF-I also indicate a key role in myelin sheath formation (Carson et al., 1993; Ye et al., 1995) .