After donor and recipient informed consent forms were signed, a single blood sample should be obtained from the donor on the day of transplant extraction. Time points for recipient's samples include the day of myeloablation and again immediately prior to HSCT (set as day 0 and possibly -3 in MUDs). Serum and lysed blood samples will be collected on days 7, 14, 28, and every 21 days thereafter, through the development and progression of GVHD (where relevant). Blood samples for FACS will be collected by days 0 and 28. Sample collection is planned to end within 1 year from HSCT, or two months after appearance of symptoms of GVHD, or two months after a change is introduced in the immunosuppressive therapy.