Patients with MPS IIIA have a clinical disorder marked by severe and progressive brain disease and neurological symptoms due to the accumulation of undigested glycosaminoglycans in all cells of the body. This study will be the first in human clinical trial to explore the safety, tolerability and clinical efficacy of ex vivo gene therapy (autologous CD34+ cells transduced with a lentiviral vector containing the human SGSH gene) in MPSIIIA patients. Following treatment with the gene therapy patients will be followed up for a minimum of 3 years.