Before the gene transfer treatment, a participant's own CD34+ hematopoietic stem cells (HSCs) will have been or will be collected from the participant's blood or bone marrow. When the participant has the required number of HSCs harvested, then the participant's HSCs will be grown in tissue culture and exposed to the lentiviral gene transfer vector containing the corrective gene (VSV-G pseudotyped CL20-4i-EF1α-hγc-OPT). These gene corrected HSCs will be administered by intravenous (IV) infusion to the participant. To increase engraftment of the corrected HSCs, participants will receive a chemotherapy drug called busulfan on 2 days before the gene transfer treatment. Participants will also receive palifermin, which helps prevent mucositis, which is the main side effect from the busulfan.