Mutations or deletions of IGF1 are associated with microcephaly, sensorineural deafness, and mental retardation (Woods et al., 1996; Bonapace et al., 2003; Netchine et al., 2009; Fuqua et al., 2012), a clinical picture consistent with the phenotype of IGF-I gene knockout mice characterized by small brains, hypomyelination, and loss of certain subtypes of neurons (Powell-Braxton et al., 1993). Here, IGF1 is linked to microcephaly.