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GM2 gangliosidoses are a group of neurodegenerative lysosomal storage disorders that are characterized by the accumulation of GM2 gangliosides (GM2), leading to rapid neurological decline and death. The hydrolysis of GM2 requires the specific synthesis, processing, and combination of products of three genes—<i>HEXA</i>, <i>HEXB</i>, and <i>GM2A</i>—within the cell’s lysosomes. Mutations in these genes result in Tay-Sachs disease, Sandhoff disease, or AB-variant GM2 gangliosidosis (ABGM2), respectively. ABGM2, the rarest of the three types, is characterized by a mutation in the <i>GM2A</i> gene, which encodes the GM2 activator (GM2A) protein. Being a monogenic disease, gene therapy is a plausible and likely effective method of treatment for ABGM2. This study aimed at assessing the effects of administering a one-time intravenous treatment of single-stranded Adeno-associated virus serotype 9 (ssAAV9)-<i>GM2A</i> viral vector at a dose of 1 × 10¹⁴ vector genomes (vg) per kilogram per mouse in an ABGM2 mouse model (<i>Gm2a</i>^−/−). <i>ssAAV9-GM2A</i> was administered at 1-day (neonatal) or 6-weeks of age (adult-stage). The results demonstrated that, in comparison to <i>Gm2a</i>^−/− mice that received a vehicle injection, the treated mice had reduced GM2 accumulation within the central nervous system and had long-term persistence of vector genomes in the brain and liver. This proof-of-concept study is a step forward towards the development of a clinically therapeutic approach for the treatment of patients with ABGM2.