Adrenoleukodystrophy (ALD) is an X-linked disease affecting 1/20,000 males either as cerebral ALD in childhood or as adrenomyeloneuropathy (AMN) in adults. Childhood ALD is the more severe form, with onset of neurological symptoms between 5-12 years of age. Central nervous system demyelination progresses rapidly and death occurs within a few years. AMN is a milder form of the disease with onset at 15-30 years of age and a more progressive course. Adrenal insufficiency (Addison's disease) may remain the only clinical manifestation of ALD. The principal biochemical abnormality of ALD is the accumulation of very-long-chain fatty acids (VLCFA) because of impaired beta-oxidation in peroxisomes. The normal oxidation of VLCFA-CoA in patients' fibroblasts suggested that the gene coding for the VLCFA-CoA synthetase could be a candidate gene for ALD. Here we use positional cloning to identify a gene partially deleted in 6 of 85 independent patients with ALD. In familial cases, the deletions segregated with the disease. An identical deletion was detected in two brothers presenting with different clinical ALD phenotypes. Candidate exons were identified by computer analysis of genomic sequences and used to isolate complementary DNAs by exon connection and screening of cDNA libraries. The deduced protein sequence shows significant sequence identity to a peroxisomal membrane protein of M(r) 70K that is involved in peroxisome biogenesis and belongs to the 'ATP-binding cassette' superfamily of transporters.