Plasma FVIII activity. A chromogenic assay was used to measure FVIII activity in plasma from tail bleeds. Males and females are shown separately, revealing an apparent sex difference. F8^F represents Cre^−/− control mice that result from breeding F8^F/y Cre^+/− males of the various Cre stains with F8^F/F females. Alb, Vav1, Cdh5^(Mlia), Cdh5^(Spe), and Tek represent experimental Cre^+/− littermates. F8^KO is the stably inherited exon 17/18-deleted strain. Hepatocyte-specific Alb-Cre has no effect on FVIII levels, whereas all endothelial Cre models result in reduced plasma FVIII, culminating with a severe hemophilic phenotype in the most efficient Tek-Cre model. *P ≤ .05, **P ≤ .001. n.s., nonsignificant.