The lysosomal removal of the sulfate moiety from sulfatide requires the action of two proteins, arylsulfatase A and sphingolipid activator protein-1 (SAP-1). Recently, patients have been identified who have a variant form of metachromatic leukodystrophy which is characterized by mutations in the gene coding for SAP-1, which is also called "prosaposin." All of the mutations characterized in these patients result in (a) deficient mature SAP-1, as determined by immunoblotting after SDS-PAGE of tissue and cell extracts, and (b) decreased ability of cultured skin fibroblasts to metabolize endocytosed [14C]-sulfatide. We now report the insertion of the full-length prosaposin cDNA into the Moloney murine leukemia virus-derived retroviral vector, pLJ, and the infection of cultured skin fibroblasts from a newly diagnosed and molecularly characterized patient with SAP-1 deficiency. The cultured cells infected with the prosaposin cDNA construct now show both production of normal levels of mature SAP-1 and completely normal metabolism of endocytosed [14C]-sulfatide. These studies demonstrate that the virally transferred prosaposin cDNA is processed normally and is localized within lysosomes, where it is needed for interaction between sulfatide and arylsulfatase A. In addition, normal as well as mutant sequences can now be found by allele-specific oligonucleotide hybridization of PCR-amplified genomic DNA by using exonic sequences as primers.