Only 25-30% of patients who may benefit from HSCT have a matched related donor. An unrelated cord blood may not be available due to size or matching criteria, or if a reduced intensity regiment is recommended. The risk of severe graft vs. host disease (GVHD) and other complications is higher with unrelated donors, or partially matched related donors. At the Children's Hospital of Philadelphia (CHOP) there is extensive experience using mismatched unrelated donors or partially matched related donors with complete or partial T depletion to reduce the risk of severe GVHD.