Typical hematopoietic stem cell (HSC) gene therapy trial design. Marrow or mobilized peripheral blood cells are collected from patients, enriched for hematopoietic stem and progenitor cells via CD34⁺ selection, and exposed to retrovirus vector particles during in vitro culture. Transduced cells are then reinfused into the patient, either with or without prior myelosuppressive or ablative conditioning radiochemotherapy. Patients are then monitored for the presence of vector and corrective transgene expression in hematopoietic progeny cells.