ALDR is one of the four genes encoding an ATP Binding Cassette (ABC) hemi-transporter of the peroxisomal membrane so far identified in mammalian cells. The best known of these is X-ALD, whose dysfunction has been causally associated with X-linked adrenoleukodystrophy. ALDR and X-ALD protein product are closely related and we show here that this striking conservation is maintained at the genomic level. Although extending to a larger genomic region, the organisation of the mouse ALDR gene mirrors exactly that of X-ALD. This supports further the hypothesis that among the four known peroxisomal ABC hemi-transporters ALDRP is the most likely candidate as a modifier contributing to the phenotypic variability of X-linked adrenoleukodystrophy.