Some MCPH1 patients also exhibit growth retardation, a feature typical of many “genetic instability disorders” caused by mutations in DNA repair proteins, such as the Seckel syndrome (caused by mutations in Ataxia-Telangiectasia and Rad3-related (ATR)) and the Nijmegen breakage syndrome (NBS, caused by mutation in NBS1, Nbn) [19]. This evidence concerns the gene XRCC6P5 and Nijmegen breakage syndrome.