INS1202 is an AAV9 shRNA construct targeting human SOD1 that is being evaluated for the potential treatment of amyotrophic lateral sclerosis (ALS). SOD1G93A mice were treated with INS1202 or vehicle via intracerebroventricular (ICV) delivery at postnatal day (p)1 and assessed for disease onset and survival, motor skills and muscle force. Plasma neurofilament levels, a validated predictive biomarker of efficacy, were evaluated at pre-symptomatic and end-stage time points; SOD1 levels, motor neuron numbers and glial activation were analyzed in spinal cords collected at disease end-stage. Treatment with INS1202 in SOD1G93A mice significantly delayed disease onset, as seen by consistent gain and maintenance of body weight throughout the study, compared to the early weight loss seen in mice treated with vehicle control. INS1202 treatment resulted in a significant increase in muscle function in both the rotarod test, which measures motor coordination and endurance, and hind-limb muscle force testing by the in vivo Aurora Scientific system. Additionally, SOD1G93A mice treated with INS1202 as compared to those treated with vehicle exhibited significantly reduced levels of plasma neurofilaments, which were similar to levels detected in wildtype mice at both pre-symptomatic (p50) and end stage (p120±10). When vehicle-treated SOD1G93A mice were euthanized at humane end-stage, mice treated with INS1202 were also sacrificed, despite lack of disease symptoms, for histological and protein analyses. INS1202 treatment reduced the levels of both total human SOD1 and misfolded SOD1 by 50-60% in the spinal cord, which resulted in significant rescue of motor neuron numbers, choline acetyltransferase (ChAT) protein expression, ionized calcium-binding adaptor molecule 1 (Iba1) and glial fibrillary acidic protein (GFAP) staining intensity to levels observed in wildtype mice, indicating a complete rescue of neurodegeneration and glial activation. Taken together, treatment with INS1202 resulted in delayed disease onset, improved muscle function, and amelioration of hallmarks of neurodegeneration in SOD1G93A mice, thus supporting the functional efficacy of INS1202 in mice.