IGF1 and microcephaly: Later, genetic studies in short children showed that absent or decreased expression of IGF-1 leads to severe pre- and post-natal growth failure, and microcephaly (6,7,8), while heterozygous (or compound heterozygous hypomorphic) mutations or deletions of IGF1R lead to a variable degree of pre- and post-natal growth failure and microcephaly (9,10,11).