The peroxisomal membrane contains several ATP-binding cassette (ABC) transporters, ABCD1–4 that are known to be present in the human peroxisome membrane (1). All four proteins are ABC half-transporters, which dimerize to form an active transporter (1). A mutation in the ABCD1 causes X-linked adrenoleukodystrophy (X-ALD), a peroxisomal disorder which affects lipid storage (2,3). ABCD2 in mouse, is expressed at high levels in the brain and adrenal organs, which are adversely affected in X-ALD (4). The peroxisomal membrane comprises 2 quantitatively major proteins, PMP22 and ABCD3 (5). ABCD3 is associated with irregularly shaped vesicles which may be defective peroxisomes or peroxisome precursors (5). ABCD4 localizes to peroxisomes (1). The genes which encode ABCD1–4 map to human chromosome Xq28, 12q11-q12, 1p22-p21 and 14q24.3, respectively (3,6–8). ABCB7 is a half-transporter involved in the transport of heme from the mitochondria to the cytosol and maps to human chromosome Xq13.1-q13.3 (9).