ATR and microcephalic primordial dwarfism: For example, MCPH is a clinical feature of both Seckel syndrome, which is caused by mutations in the ataxia-telangiectasia and Rad3-related (ATR) gene, and Nijmegen breakage syndrome, linked to mutations in the MRE11-RAD50-NBS1 (MRN) complex component NBS13,4.