The c.2624T>C variant in SCN3A is a missense variant predicted to cause substitution of isoleucine by threonine at amino acid 875 (p.Ile875Thr). The variant has been identified as a de novo occurrence with confirmed parental relationships in at least 4 individual(s) with DEE (PMIDs: 29740860, 29466837) (PS2_Very Strong) and as a de novo occurrance with unconfirmed parental relationship in a least 2 individuals with DEE (PMIDs: 29286531, 28191890) (PM6_Moderate), with additional case evidence available in the literature. The variant is absent from gnomAD v2.1.1 (PM2_Supporting). Studies have observed that when expressed in mammalian cells in culture, this variant causes a gain-of-function effect on the mutant channels as evidenced by a significant leftward/hyperpolarized shift in the voltage dependence of activation, an increase in the persistent/inactivating current, and a rightward/depolarizing shift in the voltage of inactivation (PS3; PMID: 29466837). In summary, this variant meets the criteria to be classified as pathogenic for autosomal dominant developmental and epilepstic encephalopathy based on the ACMG/AMP criteria applied, as specified by the ClinGen Epilepsy Sodium Channel VCEP: PS2_VS, PM6_Moderate, PM2_Supporting, PS3. (Version 1; July 7, 2023).