Inclusion criteria: * Age: 18 months up to 60 months (5th birthday); * Onset of clinical manifestations related to mucopolysaccharidosis type IIIB (MPSIIIB); * NAGLU activity in peripheral blood cell and/or cultured fibroblast extracts of less than 10% of controls; * Patient affiliated to, or covered by a French social security regimen, or European patients with European Health Insurance Card; * Family understanding the procedure and the informed consent; * Signed informed consent by both parents or legal representative; * Vital laboratory parameters within normal range. Exclusion Criteria: * Presence of brain atrophy on baseline MRI judged on a cortico-dural distance of more than 0.6 cm; * Any condition that would contraindicate general anesthesia; * Any other permanent medical condition not related to MPSIIIB that could contraindicate the study participation; * No independent walking (ability to walk without help); * Any medication aiming at modifying the natural course of MPSIIIB given during the 6 months before vector injection (sleep and mood regulators are accepted); * Any condition that would contraindicate treatment with Modigraf®, Cellcept® and prednisolone (Solupred® and Solumedrol®).