The clinical phenotype of individuals with homozygous IGF1 defects consists of severely reduced pre- an postnatal growth, extreme microcephaly, sensorineural deafness, failure to thrive, and poor feeding during infancy and early childhood, while those with heterogeneous defects in IGF1 or IGF1R show a similar but less severe phenotype [41–43]. Here, IGF1 is linked to microcephaly.