Metachromatic leukodystrophy is an autosomal recessive inherited lysosomal storage disease. It can be caused by mutations in two different genes, the arylsulfatase A and the prosaposin gene. These genes encode two proteins that are needed for the proper degradation of cerebroside sulfate, a glycolipid mainly found in the myelin membranes. Deficiency of arylsulfatase A or of a proteolytic product of prosaposin leads to the accumulation of cerebroside sulfate, which causes a lethal progressive demyelination. Mutations in the arylsulfatase A gene are far more frequent than those of the prosaposin gene. So far 3 1 amino acid substitutions, one nonsense mutation, three small deletions, three splice donor site mutations, and one combined missenseisplice donor site mutation have been identified in the arylsulfatase A gene. Two of these mutant alleles are frequent, accounting for about one-half of all mutant alleles, whereas the remainder are heterogeneous. Amino acid substitutions cluster in exons 2 and 3, a region that shows a high degree of conservation among sulfatases of different function and origin. Different mutations are associated with phenotypes of different severity, but there is a remarkable variability of severity when patients with identical genotypes are compared. Demonstration of an arylsulfatase A deficiency is not a proof of metachromatic leukodystrophy, since a substantial deficiency without any clinical consequences is frequent in the general population. This deficiency is caused by an arylsulfatase A allele, which due to certain mutations encodes greatly reduced amounts of functional enzyme. However, these amounts are sufficient to sustain a normal phenotype. In the diagnosis and genetic counseling, these deficiencies must be differentiated from those causing metachromatic leukodystrophy. So far only six patients with mutations in the prosaposin gene have been described, in which three defective alleles two with amino acid substitutions and one with a 33-bp insertion have been identified.