Antithrombin Cambridge, 384 Ala to Pro: a new variant identified using the polymerase chain reaction

An antithrombin III variant was identified in the plasma of a female patient with a history of recurrent thromboses. The variant was shown to have normal antigenic levels but reduced heparin and progressive inhibitory activity consistent with an abnormality affecting function at the reactive centre. Polymerase chain reaction amplification of exon 6 of the gene with direct sequencing showed a point mutation resulting in the substitution of a proline for alanine at position 384. This substitution will predictably alter the conformation of the peptide loop containing the reactive centre of the molecule.