Fourteen patients have been enrolled across three dose cohorts (Cohort 1, 5x10¹² vg/kg, n=3; Cohort 2, 1x10¹³ vg/kg, n=3; Cohort 3, 3x10¹³ vg/kg, n=8). Cohorts 1 and 2 have completed 24 months follow-up, and Cohort 3 patients were followed for a median of 19.6 months (range 11.3-26.5 months). Intravenous administration of ABO-102 was well tolerated, with no serious drug-related adverse events. ABO-102 treatment was associated with rapid, dose-dependent reduction in CSF heparan sulfate (HS), sustained for the duration of the observation period in all patients. CSF HS levels have reached the lower level of quantitation of the assay by Day 180 post-dosing in Cohort 3 patients. A rapid (day 30 post-treatment) and sustained reduction in liver volume was also measured post ABO-102 treatment and was approximately normal size for height and weight in several patients. Neurocognitive function measured by the Mullen scale (primary endpoint) tracked along the normal range in patients treated with the highest dose at younger age (≤30 months), with a follow up of 12-18 months. Several other patients showed signs of cognitive stabilization.