SPTAN1 mutations have been reported in association with autosomal dominant early infantile epileptic encephalopathy 5. Individuals present with early-onset seizures and profound intellectual disability. Recent reports suggest a wider spectrum with later-onset seizures and milder developmental delay. Here we describe two patients with loss-of-function variants in SPTAN1. One patient has ataxia and mild intellectual disability stemming from a de novo homozygous p.(Gln1448Pro) variant associated with uniparental disomy 9. The second patient, carrying a heterozygous p.(Asn1839del) allele, exhibits more substantial motor issues, developmental delay, and seizures. Ectopically expressed wild-type or variant-containing forms of sptan1 in zebrafish indicate both variants create loss-of-function alleles, with the p.(Gln1448Pro) likely being hypomorphic. This conclusion is supported by reduced protein abundance and localization of Sptan1 variants in axons of developing embryos. Further, unlike wild-type sptan1, analysis of the p.(Gln1448Pro) variant showed it failed to restore voltage-gated sodium channel localization in sptan1-null axons. Additional behavioral analyses show supplementation with the amino acid D-aspartate improved motility in sptan1-null zebrafish, supporting its use for α-II spectrin-associated motor dysfunction.