The c.1301T>G (NM_000488.4) variant in SERPINC1 is a missense variant predicted to cause substitution of phenylalanine by cystenine at amino acid 434 (p.Phe434Cys). This variant has been reported in 1 proband meeting an antithrombin activity level of < 0.8 IU/mL with repeat testing overtime (PS4_Supporting; PMID:1469094). This variant is absent from gnomAD v3.1.2 and v2.1.1 (PM2_Supporting). The computational predictor REVEL gives a score of 0.921, which is above the threshold of 0.6, evidence that correlates with impact to SERPINC1 function (PP3). Another missense variant c.1302C>A (p.Phe434Leu) (ClinVarID:2202884) in the same codon has been classified as likely pathogenic for autosomal dominant antithrombin III deficiency by the ClinGen Thrombosis VCEP (PM5_Supporting). In summary, this variant meets the criteria to be classified as uncertain significance due to insufficient evidence for autosomal dominant antithrombin III deficiency based on the ACMG/AMP criteria applied, as specified by the ClinGen Thrombosis VCEP: PP3, PM2, PM5, PS4_Supporting. (ClinGen Thrombosis Expert Panel Specifications to the ACMG/AMP Variant Interpretation Guidelines for SERPINC1 Version 1.0.0; date of approval)