Functional outcome of a novel SLC29A3 mutation identified in a patient with H syndrome

► We report a new case of H syndrome bearing a novel mutation in the SLC29A3 gene. ► His clinical phenotype is more severe than expected. ► We have studied key biochemical issues in samples derived from this patient. ► We have assessed mitochondrial function and mtDNA content under basal and challenging conditions. ► The pathogenesis of H syndrome is not associated with mitochondrial toxicity.