ECP involves the isolation of peripheral blood buffy coat cells, ex vivo exposure of the cells to 8-methoxypsoralen (8-MOP) and ultraviolet-A radiation, and subsequent re-infusion of the treated cells to the patient. The combination of 8-MOP and PUVA results in DNA crosslinks and causes apoptosis. However, the direct induction of lymphocyte apoptosis is unlikely to account for the clinical efficacy of ECP given that less than 10% of circulating leukocytes are exposed to PUVA during ECP.