Inclusion Criteria: * Confirmation of two pathogenic variants in the IGHMBP2 gene from a CLIA-certified lab * Pre-ambulant (not yet walking and less than 18 months) or ambulant (as defined by the ability to walk 10 meters without assistance) or non-ambulant (inability to walk more than 10 meters unassisted) * Ability to cooperate with functional assessments as per PI's discretion Exclusion Criteria: * Prior participation in a gene or cell therapy program for any kind. * Immunizations of any kind in the month prior to the study. * Active infection based on clinical observations * Serological evidence of HIV infection, or Hepatitis B or C infection * Diagnosis of (or ongoing treatment for) an autoimmune disease * Persistent leukopenia or leukocytosis (WBC ≤ 3.5 10\^3/μL or ≥ 20.0 10\^3/μL) or an absolute neutrophil count \< 1.5 10\^3/μL * Abnormal liver function as indicated by an elevated GGT (\>2X normal if no other laboratory abnormalities), bilirubin and/or abnormal PT/INR * Concomitant illness or requirement for chronic drug treatment that in the opinion of the PI creates unnecessary risks for gene transfer * AAV9 binding antibody titers \> 1:50 as determined by ELISA immunoassay performed by Athena Diagnostics * Abnormal laboratory values in the clinically significant range, based upon normal values in the Nationwide Children's Hospital Laboratory * Diagnosis of any other systemic illness that increases the risk of gene transfer per the PI's opinion; Has a medical condition or extenuating circumstance that, in the opinion of the PI, might compromise the subject's ability to comply with the protocol required testing or procedures or compromise the subject's well-being, safety, or clinical interpretability * Any requirement for immune modulatory therapy and for which it would be unsafe for the subject to undergo an appropriate wash out period * Contraindication for intrathecal injection * A positive JCV antibody test of \>0.40