Although the accumulation of VLCFAs in X-ALD patients pointed to a role of ABCD1 in transporting VLCFAs across the peroxisomal membrane, the actual transport mechanism has long remained unclear, mainly because the hydrophobic nature of VLCFAs makes classical in vitro transport experiments with liposomes difficult. The gene discussed is ABCD1; the disease is X-linked adrenoleukodystrophy.