Women who are heterozygous carriers of a hemophilic mutation may have low levels of the implicated clotting factor. This depends on the ratio of inactivation of the hemophilic and normal X chromosomes, a random process that occurs early during embryonic development. If there is markedly skewed inactivation of the X normal chromosome, the plasma level of FVIll or FIX may be correspondingly reduced, and the carrier female may have evidence of bleeding disorder. Thus FVIll or FIX levels should be determined in all potential carrier females before adolescence when they might first manifest with menorrhagia. A low plasma level of FVIII or FlX is predictive of the carrier state, and the lower the level, the more probable the carrier diagnosis.