The dosage effect of platelets expressing FVIII on phenotypic correction. Different proportions of mixed BM mononuclear cells from 2bF8^tg+/− (20%, 10%, 5%, and 1% of 2bF8^tg+/−) and FVIII^null (80%, 90%, 95%, and 99% of FVIII^null) were prepared and transplanted into lethally irradiated immunized FVIII^null recipients. (A) PCR detection of 2bF8 transgene. DNA was purified from peripheral white blood cells. A 0.292-kb fragment from 2bF8 transgene and a 0.207-kb fragment from the wild-type allele were amplified by PCR. (Left panel) PCR results from recipients before BM transplantation. (Right panel) Results after BM transplantation. Shown is one representative experiment that was performed 4 times. (B) Quantitative real-time PCR determined the average copy number of 2bF8 transgene per cell in BMT recipients. (C) Tail clip survival test assessed phenotypic correction in recipients. There is no significant difference between those groups. Data shown are summarized from 2 BM transplantation trials. These data show that having even a small proportion of platelets that contain FVIII still improves hemostasis in hemophilic mice with preexisting immunity.