The kidney plays a crucial role in maintaining fluid and electrolyte homeostasis. In adults, selective filtration by the renal glomerulus produces more than 150 liters of ultrafiltrate every day; 99% of which must be reabsorbed by highly specialized segments of the renal tubule. Impaired function of the first post- glomerular segment, the proximal tubule, can cause life-threatening losses of fluids, electrolytes, and low-molecular-weight nutrients. Such dysfunction, known as renal Fanconi’s syndrome most often occurs in children as part of multisystem metabolic diseases such as cystinosis, Dent’s disease, the Fanconi-Bickel syndrome, the oculocerebrorenal syndrome (Lowe’s syndrome), tyrosinemia type I, Wilson’s disease, fructose intolerance, galactosemia, the ARC (arthrogryposis, renal dysfunction, and cholestasis) syndrome, and mitochondrial disorders. In both children and adults, acquired Fanconi’s syndrome may result from toxic effects of anti-human immunodeficiency virus drugs, heavy metals, antibiotics, valproic acid, glue sniffing, or exposure to suramin, fumaric acid, or ifosfamide.1,2 Finally, there have been rare cases of severe, isolated renal Fanconi’s syndrome in which the molecular, biochemical, and cellular defects remain unknown.3 Elucidation of these defects might provide a better understanding of the mechanisms of tubular reabsorption and lead to new therapeutic interventions.One possible cause of renal Fanconi’s syndrome involves impaired mitochondrial function, because the high metabolic activity of the proximal tubule makes it particularly susceptible to restrictions in energy output.4 Mitochondrial production of ATP, essential for generating the energy dependent ion gradients that drive renal tubular reabsorption, is impaired in acute kidney injury.5 We have recently gained further insight into a mechanism of impaired tubular function by identifying a defective mithochondrial protein through clinical and genetic characterization of a family with isolated autosomal dominant Fanconi’s syndrome. We linked the phenotype of this family’s Fanconi’s syndrome to a single locus on chromosome 3q27, where a heterozygous missense mutation in EHHADH segregated with the disease. The p.E3K mutation created a new mitochondrial targeting motif in the N-terminal portion of EHHADH, an enzyme that is involved in peroxisomal oxidation of fatty acids and is expressed in the proximal tubule. Immunocytofluorescence studies showed mistargeting of the mutant EHHADH to mitochondria. Studies of proximal tubular cells revealed impaired mitochondrial oxidative phosphorylation and defects in the transport of fluids and a glucose analogue across the epithelium. 1H-NMR spectroscopy showed elevated levels of mitochondrial metabolites in urine from affected family members. Ehhadh knockout mice showed no abnormalities in renal tubular cells, a finding that indicates a dominant negative nature of the mutation rather than haploinsufficiency. Mistargeting of peroxisomal EHHADH disrupts mitochondrial metabolism and leads to renal Fanconi’s syndrome; this indicates a central role of mitochondria in proximal tubular function. The dominant negative effect of the mistargeted protein adds to the spectrum of monogenic mechanisms of Fanconi’s syndrome.