Expression of the human blood-clotting factor VIII (FVIIIHemophilia A is a bleeding disorder that affects approximately 1 in 10,000 males and is caused by a deficiency of functional blood coagulation factor VIII (FVIII). Protein replacement therapy has been effective and has resulted in both a dramatic increase in life expectancy and a vast improvement in the quality of life (41). However, therapy with plasma-derived FVIII has resulted in transmission of several human viruses, such as human immunodeficiency virus and hepatitis viruses (39). The ideal therapy, therefore, would be independent of blood-derived products. The recent use of recombinant-DNA-derived FVIII for treatment of hemophilia A (35) is a major step in that direction, but extensive use of this product is still problematic because of the high cost of recombinant FVIII. Therefore, somatic gene therapy is an appealing alternative for hemophilia treatment (15,24,42).As a first step in this direction, gene therapists have focused on the development of retroviral vectors and ex vivo strategies for the transfer and expression of the human blood-clotting FVIII cDNA. It has been shown previously that the FVIII proteins secreted by various cell types are fully functional (25,27). However, the titers of the retroviruses carrying the FVIII cDNA (Ͻ10 4 CFU/ml) are much lower than those routinely obtained with vectors that carry other genes (Ͼ10 6 CFU/ml), and the amounts of protein that infected cells secrete are smaller (22,24,25,27,32,36).Several causes have been named to explain the low level of FVIII expression, such as inefficient intracellular transport of FVIII, inefficient FVIII secretion, and extreme susceptibility of this clotting factor to proteolytic degradation (11,12,29,30,40). In addition, reduced FVIII mRNA accumulation has been observed in various cell types, like Swiss 3T3, primary human skin fibroblasts, Rat2 fibroblasts, and Chinese hamster ovary (CHO)-derived cell lines (22,30,36). Interestingly, treatment of CHO-derived cell lines and Rat2 fibroblasts with sodium butyrate raised their steady-state FVIII mRNA and protein levels (11,22,29). The mechanism by which butyrate exerts this effect is still unclear.Lynch and coworkers (36) demonstrated that inhibition of FVIII mRNA accumulation in retrovirus-packaging and trans-* Corresponding author. Mailing address: