The IM and ABC domains of the proteins of this family are fused into a single polypeptide chain (IM-ABC). In yeast, two homologous peroxisome-associated proteins PXA1 and PXA2 form heterodimers and when inactivated, determine an impaired growth on oleic acid and a reduced ability to oxidize oleate ( Pubmed : 7597071 ) . The adrenoleukodystrophy protein ALDp (ABCD1) is an ABC transporter in the human peroxisome membrane. It is defective in X chromosome-linked adrenoleukodystrophy (ALD), a neurodegenerative disorder with impaired peroxisomal oxidation of very long chain fatty acids ( Pubmed : 8040304 ) . Three other proteins, highly similar to ABCD2, were identified in the human genome: ALDR (ABCD2), PMP70 (ABCD3) and PMP69 (ABCD4). A mutated form of PMP70 was associated with certain manifestations of Zellweger syndrome, a group of genetically heterogeneous disorders affecting peroxisome biogenesis ( Pubmed : 1301993 ) . The actual function of these transporters is unknown, but it has been proposed that they could export into peroxisomes long chain fatty acids or the enzyme(s) responsible for their degradation ( Pubmed : 10856710 ) . Interestingly, several proteins strongly similar to ALDp over the entire sequence length were detected in bacteria, but their functions were not yet investigated.