There is currently no known effective therapy for patients with acute graft vs host disease that's refractory (nonresponsive) to steroid therapy. As stated earlier,the overwhelming majority of these patients may ultimately die from infection. The incidence of acute GvHD that requires intervention, is higher for unrelated donor transplants, the most common treatment option available, and therefore, these patients are at higher risk for treatment related complications from GVHD. Approximately 20,000 unrelated donor transplants are performed each year. The magnitude of this problem then is significant for patients who otherwise might be cured of their blood cancer or disease.