ABCG2 and X-linked adrenoleukodystrophy: In addition to the documented role for the ABC transporter in VLCFA metabolism and ALD, a role for the acyl-CoA synthases (ACSs) that function immediately upstream of ABC transporters in fatty acid (FA) metabolism has long been contemplated, as decreased ACS activity is another biochemical hallmark of ALD (Hashmi et al., 1986; Lazo et al., 1988; Wanders et al., 1988).