The c.89T>A (NM_000488.4) variant in SERPINC1 is a missense variant predicted to cause substitution of valine by glutamic acid at amino acid 30 (p.Val30Glu; legacy p.V-3E; also known as the founder variant antithrombin Dublin), so the BS1 code is excluded. This sequence variant has been previously reported in patients with transient antithrombin deficiency, who present with a history of reduced antithrombin activity with at least one reported normal antithrombin activity level (PMID: 27975105; PMID: 27098529; PMID: 34800304) meeting PS4_Very Strong. Large cohort analysis demonstrate carriers of this variant have a 2.3-fold increased risk of thrombosis compared to the general population (PMID: 25772935; PMID: 27098529). Additionally, this variant has been report in at least 4 meioses across 8 families meeting PP1_Moderate. Functional studies performed in mammalian cells demonstrated this variant creates a new signal peptidase cleavage site two amino acids downstream of the normal site, leading to a truncated mature protein missing two amino acids at the N-terminus (PMID: 1977621; PMID: 27098529). The truncated protein has an increased propensity to form intracellular polymers, which hinders antithrombin secretion (PMID: 27098529; PMID: 28229161). Transient environmental factors or other predisposing stress events may exacerbate this polymerization and intracellular retention, contributing to the transient deficiency observed in affected individuals (PMID: 27098529). In summary, this variant meets the criteria to be classified as pathogenic for autosomal dominant antithrombin deficiency based on the ACMG/AMP criteria applied, as specified by the ClinGen Thrombosis VCEP: PS4_Very Strong, PP1_moderate, PS3.