GVHD is a serious and sometimes fatal side effect of stem cell transplant. GVHD occurs when the new donor cells (graft) recognizes that the body tissues of the patient (host) are different from those of the donor. When this happens, cells in the graft may attack the host organs, primarily the skin, liver, and intestines. The number of occurrences and harshness of severe GVHD depends on several factors, including the degree of genetic differences between the donor and recipient, the intensity of the pre-treatment conditioning regimen, the quantity of transplanted cells, and the recipient's age. In recipients of mismatched family member or matched unrelated donor stem cell transplants, there is a greater risk of GVHD so that 70-90% of recipients of unchanged marrow will develop severe GVHD which could include symptoms such as marked diarrhea, liver failure, or even death.