Abstract
The relation between functional antihemophilic factor (AHF) activity and AHF-like antigen was studied in the plasma of 25 known carriers of hemophilia. In 23 cases, this relationship was significantly different from that in normal women, at the 99% limit of confidence. In contrast, among families in which only one case of hemophilia had occurred, only five of nine mothers could be identified as carriers. This observation suggests that in some instances the hemophilia arose from a newly mutant gene. The data are consistent with the hypothesis that the proportion of antigen to AHF activity in carriers is determined by random activation or inactivation of the X chromosome.