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Heterozygous truncating loss-of-function (LoF) variants in NEK1 are a known cause of amyotrophic lateral sclerosis (ALS). NEK1 encodes the pleiotropic serine/threonine kinase NIMA-related kinase 1, and prior in vitro studies have implicated kinase dysfunction as the principal pathogenic mechanism underlying NEK1-associated ALS. However, bona fide pathogenic missense variants causally linked to ALS have not previously been reported, leaving this hypothesis unconfirmed. Here, we identify a rare NEK1 missense variant, p.N598S, that co-segregates with disease in a familial ALS pedigree and is enriched in European ALS cohorts. This variant exhibits normal protein expression levels, indicating a functional rather than quantitative defect. Using isogenic human motor neurons, we directly compared the effects of p.N598S with those of the ALS-associated truncating variant p.R812* to delineate disease mechanisms. The p.N598S variant induced pathological phenotypes consistent with NEK1 haploinsufficiency, including increased susceptibility to DNA damage, increased apoptosis, ciliary dysmorphia, and nucleocytoplasmic translocation of TDP-43. Importantly, p.N598S impaired NEK1 kinase activity, and pharmacological inhibition of NEK1 recapitulated the cellular phenotypes observed in both p.N598S- and p.R812*-mutant motor neurons. Collectively, these findings provide strong genetic and functional evidence for a disease-causing role of NEK1 kinase disruption in NEK1-ALS. Our findings provide immediate diagnostic and therapeutic implications, particularly for the functional interpretation of missense variants of uncertain significance and the development of targeted treatment strategies.