Sickle cell disease is a blood disorder caused by a single mutation in the beta-globin gene that results in the substitution of one amino acid. This small error is enough to change the properties of the protein: when “sickled,” the blood’s hemoglobin dumps its oxygen in tissues, and it tends to stick to itself. This makes red blood cells abnormally rigid, adhesive, and distorted into the hallmark sickle shapes that characterize the disease. Now, Harvard Medical School researchers have used a gene therapy to cure sickle cell disease in a mouse model. The achievement, announced in the Dec. 14, 2001, issue of the journal Science, marks a breakthrough in what initially seemed like a simple task: to cure a rather straightforward genetic defect by introducing a new gene into the stem cells of bone marrow. The effort proved more arduous than expected, forcing scientists to confront the complexities of stem cell biology, virology, gene expression, and protein biochemistry. The research team was led by Philippe Leboulch, Harvard Medical School assistant professor of medicine at Brigham and Women’s Hospital.