This clinical trial studies biomarkers in predicting response in patients with
graft-versus-host disease (GVHD) undergoing extracorporeal photopheresis (ECP). ECP treats
the patient's blood with ultraviolet light outside the body and kills the white blood cells
before returning blood back into the patient's body. Studying samples of blood from patients
with GVHD may help doctors identify and learn more about biomarkers related to GVHD.