BMT of 2bF8 genetically modified hematopoietic stem cells rescues the severe spontaneous hemorrhagic phenotype in Dahl-FVIII^−/−rats. BM nucleated cells from 2bF8 transgenic rats were transplanted into 8- to 16-week-old Dahl-FVIII^−/− rats preconditioned with 950 cGy total-body irradiation. Animals were closely monitored for bleeding episodes. After BM reconstitution, blood samples were collected from the ventral tail artery at various time points. Platelets and plasma were isolated for FVIII assays. Functional FVIII expression levels were determined by using a modified chromogenic assay of platelet lysates and plasmas. B-domain–deleted rhFVIII (Xyntha) was used as a reference standard for FVIII:C assays. (A) A schematic diagram for BMT. (B) Platelet FVIII expression levels in Dahl-FVIII^−/− recipients after receiving BMT from 2bF8^tg rats. Data shown are the average platelet FVIII:C calculated from 3 time points after BMT. Platelets isolated from Dahl-2bF8^tg donors and Dahl-WT were used as controls. (C) Plasma FVIII expression levels in Dahl-FVIII^−/− rats after receiving BMT from 2bF8^tg animals. (D) Platelet counts in peripheral blood from recipients at various time points after BMT. Platelet counts were analyzed by using the Vet ABC Hematology Analyzer (scil animal care company, Gernee, IL). (E) Occurrence of spontaneous bleeding episodes in Dahl-FVIII^−/− recipients after receiving BMT from 2bF8^tg rats. The bleeding event in Dahl-FVIII^−/− rats without BMT was used as a control. (F) Frequency of spontaneous bleeding episodes in 2bF8^tg-BMT recipients by the age of 28 weeks compared with Dahl-FVIII^−/− rats.