The c.1274G>C variant in SERPINC1 is a missense variant predicted to cause substitution of arginine by proline at amino acid 425 (p.Arg425Pro). This variant has been reported in one proband meeting an antithrombin activity level of < 0.8 IU/mL with a family history of antithrombin activity levels of < 0.8 IU/mL (PS4_Supporting; PMID:3828226, 2722864). The variant has been reported to segregate with hereditary antithrombin deficiency in seven affected meioses from the proband's family (PP1_Moderate; PMID: 3828226). The computational predictor REVEL gives a score of 0.878, which is above the threshold of 0.6, evidence that correlates with impact to SERPINC1 function (PP3). Another missense variant c.1274G>A (p.Arg425His) (ClinVarID:18019) in the same codon has been classified as pathogenic for hereditary antithrombin deficiency by the ClinGen Thrombosis VCEP (PM5). This variant is absent from gnomAD v2.1.1 and v3.1.2 (PM2_Supporting). In summary, this variant meets the criteria to be classified as likely pathogenic for autosomal dominant hereditary antithrombin deficiency based on the ACMG/AMP criteria applied, as specified by the ClinGen Thrombosis VCEP: PM2_Supporting, PS4_Supporting, PP3, PM5, PP1_Moderate.