Lysosomal storage diseases are typically, but not exclusively, childhood diseases resulting from a genetic deficiency in a lysosomal enzyme involved in a particular metabolic pathway that results in lysosomal accumulation of its substrate(s). Many of these enzymes can be secreted from cells genetically engineered to overexpress them and then taken up by enzyme-deficient cells and correctly targeted to lysosomes via mannose-6-phosphate receptors where they degrade the stored substrate(s). This mechanism, known as cross-correction (38), is the basis for enzyme replacement therapy which is now available for a subset of these diseases without neurological involvement. Unfortunately, the BBB prevents recombinant lysosomal enzymes infused peripherally from entering the CNS. Alternative routes of delivery and molecules are being actively investigated (see above). These diseases are particularly well suited for gene therapy as they are monogenic diseases with very well established genotype–phenotype correlations, and the cross-correction mechanism makes it possible to devise gene delivery strategies to supply essentially the entire CNS with therapeutic levels of these enzymes. Two main approaches have shown dramatic therapeutic effects in animal models of lysosomal storage diseases, namely intraparenchymal infusion of recombinant viral vectors (39–46), and bone marrow transplantation with ex vivo lentivirus vector modified-autologous HSCs (see article by Biffi, Cartier and Aubourg in this issue)