Summ a r ySickle cell disease results from a homozygous missense mutation in the β-globin gene that causes polymerization of hemoglobin S. Gene therapy for patients with this disorder is complicated by the complex cellular abnormalities and challenges in achieving effective, persistent inhibition of polymerization of hemoglobin S. We describe our first patient treated with lentiviral vector-mediated addition of an antisickling β-globin gene into autologous hematopoietic stem cells.Adverse events were consistent with busulfan conditioning.Fifteen months after treatment, the level of therapeutic antisickling β-globin remained high (approximately 50% of β-like-globin chains) without recurrence of sickle crises and with correction of the biologic hallmarks of the disease.(Funded by Bluebird Bio and others; HGB-205 ClinicalTrials.govnumber, NCT02151526.)S ickle cell disease is among the most prevalent inherited monogenic disorders.Approximately 90,000 people in the United States have sickle cell disease, and worldwide more than 275,000 infants are born with the disease annually. 1,2Sickle cell disease was the first disease for which the molecular basis was identified: a single amino acid substitution in "adult" β A -globin (Glu6Val) stemming from a single base substitution (A→T) in the first exon of the human β A -globin gene (HBB) was discovered in 1956. 3Sickle hemoglobin (HbS) polymerizes on deoxygenation, reducing the deformability of red cells.Patients have intensely painful vaso-occlusive crises, leading to irreversible organ damage, poor quality of life, and reduced life expectancy.Hydroxyurea, a cytotoxic agent that is capable of boosting fetal hemoglobin levels in some patients, is the only disease-modifying therapy approved for sickle cell disease. 4llogeneic hematopoietic stem-cell transplantation currently offers the only curative option for patients with severe sickle cell disease. 5,6However, fewer than 18% of patients have access to a matched sibling donor. 7,8Therapeutic ex vivo gene transfer into autologous hematopoietic stem cells, referred to here as gene therapy, may provide a long-term and potentially curative treatment for sickle cell disease. 9e previously reported proof of effective, sustained gene therapy in mouse mod-

Gene Therapy in a Patient with Sickle Cell Disease