Factor × Roma: a congenital factor × variant defective at different degrees in the intrinsic and the extrinsic activation

A factor × molecular variant was identified in a 13-year-old girl affected by a bleeding tendency. Factor × antigen levels and activation by Russel's viper venom (tested both by clotting and amidolytic assays) were normal. Factor × crossed immunoelectrophoresis was found to be identical to that of the control plasma. Factor × functional activity was low (3% of the normal) if tested by PTT-derived assays, whereas it was found at intermediate levels (about 30–50% of the normal) if measured by prothrombin time-derived assays. The defect in the extrinsic activation was more clearly disclosed using as activating agent thromboplastin from ox brain. The factor × of the patient was completely adsorbed by aluminium hydroxide. The parents of the propositus (first degree cousins) showed factor × functional levels compatible with a condition of heterozygosity for the abnormality. This factor × molecular variant appears different from the other ones so far described and was named ‘Factor × Roma’.