Treatment involves the patient ingesting 8-methoxsalen (8-MOP) by mouth approximately two hours prior to each photopheresis session. The photopheresis instrument will separate the patient's white blood cells and some plasma from other blood components and return the majority of blood components to the patient. The separated white blood cells and plasma will be exposed to UVA (Ultraviolet A) light while outside the body. The UVA light photoactivates the 8-MOP drug which is now located in the DNA of the collected cells. After this exposure, the patients photoactivated white blood cells will be returned to the patient by venous access.