Three members of a San Antonio, Texas, family were identified with prothrombin activity levels half the normal level but to have normal levels of antigen. All exons of the prothrombin gene from the proband were sequenced. A G-to-A mutation at nucleotide 7543 was found that resulted in the substitution of His for Arg at residue 320. The Arg320-Ile321 bond is 1 of 2 sites in prothrombin cleaved by Factor Xa in the prothrombinase complex to form thrombin. Substitution of His for Arg at this site resulted in the blockage of Factor Xa cleavage, forming a dysfunctional molecule. The proband, her mother, and her maternal aunt were found to be heterozygous for this mutation. This is the first known observation of an amino acid substitution at this site that resulted in dysprothrombinemia. (Blood. 2000;95:711-714)