This clinical trial aims to test a new gene therapy in adults (18 to 35 years old) with beta thalassemia who require regular red cell transfusions. The study will evaluate the safety and efficacy of genetically engineered blood stem cells using a novel lentiviral vector that produces a high levels of a healthy beta globin. Several tests and procedures such as review of medical history, blood and urine tests, bone marrow aspirate/biopsy, heart ultrasound, electrocardiogram, magnetic resonance imaging (MRI) of the heart and liver, and lung tests will be done to find out if subjects are eligible to participate in this trial. If eligible, blood stem cells will be collected by apheresis. These stem cells will then have a healthy beta globin gene inserted. Subjects will then be hospitalized, and chemotherapy will be given to empty out the bone marrow. The stem cells, with the inserted gene (CHOP-ALS20), will then be given back to the subject. Subjects will be followed for side effects and to see if the inserted gene helps make hemoglobin and reduce or stop blood transfusions.