Of interest this patient exhibits one of the most severe forms of MCSZ documented, with a level of microcephaly and growth retardation comparable to patients with ataxia telangiectasia and Rad3 related-Seckel Syndrome.16 Despite this, in keeping with the functional analysis of other identified PNKP mutations,15 cells derived from this patient retain some residual PNKP activity. The gene discussed is PNKP; the disease is ataxia telangiectasia.