Single Injection of rAAV9-CMV-hNAGLUop Gene for Patients With Mucopolysaccharidosis (MPS) IIIB

The goal of this clinical trial is to learn if one infusion of rAAV9-CMV-hNAGLUop can treat patients with Sanfilippo syndrome (MPS III) 6 months or older. The main questions it aims to answer are: * Is the treatment safe? * Is there a change in enzyme activities,? * Is there a change in natural history trajectory of motor, language, feeding, adaptive and cognitive function? Participants will be asked to come to Washington University St. Louis and stay in the hospital overnight following the infusion. Partcipant and caregiver(s) will be asked to stay near the hospital for the first month and come back periodically for clinic visits.