The congenital brain malformations, microcephaly and growth delay observed in our CIAO1 and MMS19 deficiency patients are in line with the observation that loss of functional Mms19 drastically affected the growth and development of mitotic tissues in Drosophila larvae with brains showing a microcephaly phenotype.24 In addition, MMS19-beta-GEO gene trap mice are embryonically dead in the preimplantation stage.22 Furthermore, Ciao1 has been shown to regulate organ growth and to be mainly required for survival and proliferation of undifferentiated cells in Drosophila. Here, MMS19 is linked to microcephaly.