Recommended biochemical tests include very long-chain fatty acid (VLCFA) profiles, which are elevated in the plasma and tissues of patients with X-ALD [40, 47–50], and measurements of leukocyte lysosomal enzyme activity are used to investigate deficiencies in galactocerebrosidase (GALC) (Krabbe disease) [51] and arylsulfatase A (ARSA) activity (MLD) [52]. This evidence concerns the gene ARSA and X-linked adrenoleukodystrophy.