GS-100 is an AAV9 gene replacement therapy for the treatment of N-glycanase 1 (NGLY1) deficiency, an ultra-rare, recessive disorder characterized by developmental delay, intellectual disability, hyperkinetic movement disorder, elevated liver enzymes, (hypo)alacrima, and peripheral neuropathy. In pharmacology studies using Ngly1 -/- rats, intracerebroventricular (i.c.v.) administration of GS-100 showed dose-dependent biodistribution, increased hNGLY1 mRNA expression, reduced disease biomarker N-acetylglucosamine-asparagine (GNA), improved motor function, and prevented phenotype progression. Reductions in GNA within central nervous system (CNS) tissues and cerebrospinal fluid (CSF) correlated significantly with improvements, suggesting that CSF GNA may serve as a clinical biomarker of therapeutic activity. Safety studies conducted in wild-type rats and non-human primates (NHPs) indicated no adverse clinical signs, mortality, body weight changes, or negative neurobehavioral/nerve conduction effects at or near the clinical starting dose of 4e14 vector genome (vg)/individual. In addition, GS-100 biodistribution was widespread in CNS and peripheral tissues. These nonclinical data supported the Food and Drug Administration approval of an Investigational New Drug application for a first-in-human phase 1/2/3 dose-escalation clinical trial. The trial's co-primary endpoints include changes in CSF GNA biomarker levels and improvements or stabilization of motor function to assess the safety and efficacy of GS-100 administered i.c.v. in patients with NGLY1 deficiency.