Studies on patients with features suggestive of MLD, but with normal arylsulfatase A activity resulted in the confirmation that these patients were deficient in a specific sphingolipid activator protein required for the enzymatic hydrolysis of sulfatide.28 The antibodies produced in this laboratory were used to show the deficiency of this activator protein in cultured cells from the patients and to clone the activator protein gene, now called prosaposin.29, 30 That lead to the identification of mutations in the prosaposin gene.31, 32. The gene discussed is ARSA; the disease is metachromatic leukodystrophy.