CD34 and lymphopenia: These factors are T-cell depletion of the graft by ex vivo CD34+ selection or in vivo use of alemtuzumab or anti-thymocyte serum, the development of grade III-IV graft versus host disease (GVHD), the type of donor (unrelated donor, cord blood, haploidentical or HLA mismatched parent), and severe lymphopenia in the first months after allo-HCT(<0.2 × 109/L) [7,9,14,20,21].