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Diagnosis of mitochondrial disease is often a challenge because of the extreme heterogeneity of the clinical phenotype and the variety of underlying gene defects. Insight into the range of clinical phenotypes associated with a particular mitochondrial DNA mutation will facilitate better recognition of patients at risk by focused gene testing. We present a family affected by the mitochondrial m.13513G>A (p.D393N, ND5) mutation, illustrating a previously unreported degree of clinical heterogeneity, varying from mitochondrial encephalopathy with lactic acidosis and stroke-like episodes (MELAS) in a 10-year-old female, to a fatal neonatal course with metabolic acidosis and hypotonia in a younger sister, to absence of medical problems in the mother. The mutation loads ranging from 66% in the deceased neonate to 30% in the female with MELAS and 7% in the asymptomatic mother, correlated with severity of the clinical phenotype. The importance of proactive collection and storage of appropriate samples during the diagnostic work-up of an acutely ill or deceased neonate, allowing subsequent mitochondrial investigations, is hereby illustrated., (© The Authors. Developmental Medicine & Child Neurology © 2011 Mac Keith Press.)