Discordance in a pair of identical twin carriers of factor IX deficiency.

Female twin daughters of a man with hemophilia B were determined to be identical but widely discordant with respect to their degree of factor IX deficiency. The proposita had a marked deficiency (3%) of factor IX with clinical bleeding. She became pregnant. Delivery was managed with infusion of factor IX concentrates. These twins represent the second reported case of identical twins with discordant factor IX levels.