Number of VOEs before and after HSCT in patients with graft rejection. Number of VOEs before and after transplant in 20 patients with graft rejection displayed by (a) sickle hemoglobin and (b) fetal hemoglobin percentages at 2 years post transplantation. (C) Fetal hemoglobin percentages at the time of transplant vs 2 years post transplantation. HbF, fetal hemoglobin; HbS, sickle hemoglobin; HSCT, hematopoietic stem cell transplantation; VOE, vaso-occlusive event.