The introduction of PTCy on days +3 and +4 after HSCT, pioneered by the Johns Hopkins group, has greatly improved outcomes of haploidentical HSCT, resulting in low incidences of GVHD and non-relapse mortality [39] that may be attributable to PTCy-mediated protection through deletion of adoptively transferred alloreactive T cells and rapid preferential recovery of FoxP3+ regulatory T cells [40]. The gene discussed is FOXP3; the disease is graft versus host disease.