Recently, loss of function mutations in ARID1B were identified as causative for Coffin-Siris syndrome, a rare genetic condition characterized by growth deficiency, developmental delay, severe speech impairment, intellectual disability, and specific physical features including microcephaly, coarse facial features, hypertrichosis, hypoplastic or absent fifth fingernails or toenails and hypoplasia or agenesis of the corpus callosum [13]. This evidence concerns the gene ARID1B and microcephaly.