We are conducting a gene therapy clinical trial based on autologous HSC and advanced generation lentiviral vectors (LV) for patients affected by the most severe, early onset forms of the disease, based on our preclinical efficacy and safety data and on the experience we acquired from the treatment of Adenosine deaminase deficiency (ADA-SCID) patients by a similar approach. Our general goal is to demonstrate safety and efficacy of this gene therapy approach in MLD patients. We have developed and tested a clinically applicable LV containing the ARSA cDNA controlled by a constitutive eukaryotic promoter sequence. Preclinical studies aimed at assessing efficacy and safety of ARSA gene transfer using the selected vector have been conducted in the murine model of the disease and in progenitors and mature hematopoietic cells from MLD patients. The following results were obtained: