To date, MLD is diagnosed by clinical manifestations, using genetic analysis for mutations in the ARSA and PSAP genes (68), magnetic resonance imaging (MRI) of the brain (69–71) and biochemical tests of the ARSA enzymatic activity in skin fibroblasts, leukocytes and urine of patients (31, 72). The gene discussed is ARSA; the disease is metachromatic leukodystrophy.