The c.1306G>A (NM_000488.4) variant in SERPINC1 is a missense variant predicted to cause substitution of alanine by threonine at amino acid 436 (p.Ala436Thr). This variant has been reported in 2 probands meeting an antithrombin activity level of < 0.8 IU/mL with either a family history of antithrombin activity levels of < 0.8 IU/mL or an abnormal crossed immunoelectrophoresis assay demonstrating decreased antithrombin function. One more proband is reported with antithrombin activity levels of < 0.8 IU/mL but no repeat testing or family history is specified so 0.5 point is awarded (PS4_Moderate; PMID:3055413, 1469094). The variant has been reported to segregate with autosomal dominant antithrombin deficiency in 11 affected meioses from one family (PP1_Moderate; PMIDs:1469094, 3055413). The computational predictor REVEL gives a score of 0.778, which is above the threshold of 0.6, evidence that correlates with impact to SERPINC1 function (PP3). This variant is absent from gnomAD v2.1.1 and gnomAD v4.1 (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: PP1_Moderate, PM2_Supporting, PS4_Moderate, PP3.