In a murine model of sickle cell disease (SCD), recipients conditioned with low dose of the alkylating agent (busulfan) along with anti-CD40L mAb and the fusion protein, CTLA4-Ig that blocks the CD28/CTLA-4 costimulatory pathway became mixed chimeras that permitted the acquisition of normal red blood cell morphology without evidence of GVHD (30). The gene discussed is CTLA4; the disease is sickle cell disease.