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Summary: ADP-ribosylation is a post-translational modification involving the transfer of one or more ADP-ribose units from NAD+ to target proteins. Dysregulation of ADP-ribosylation is implicated in neurodegenerative diseases. In this study, genetic testing via exome sequencing was used to identify the underlying disease in two siblings with developmental delay, seizures, progressive muscle weakness, and respiratory failure following an episodic course. This identified a novel homozygous variant in the ADPRS gene (c.545A>G, p.His182Arg) encoding the mono(ADP-ribosyl) hydrolase ARH3, confirming the diagnosis of childhood-onset neurodegeneration with stress-induced ataxia and seizures (CONDSIAS) in these two children. Mechanistically, the ARH3H182R variant affects a highly conserved residue in the active site of ARH3, leading to protein instability, degradation, and, subsequently, reduced protein expression. The ARH3H182R mutant additionally fails to localize to the nucleus, which further resulted in accumulated mono-ADP ribosylated species in cells. The children’s clinical course combined with the biochemical characterization of their genetic variant develops our understanding of the pathogenic mechanisms driving CONDSIAS and highlights a critical role for ARH3-regulated ADP-ribosylation in nervous system integrity.