The neomycin resistance gene in a retroviral vector has been used to mark the marrow of patients receiving autologous bone marrow transplantation for neuroblastoma and acute myeloid leukemia. We have shown that the marker gene can be detected in the resurgent malignant cells at the time of relapse, directly demonstrating that tumorigenic cells contaminate "remission" marrow. We have also shown effective gene transfer to normal primitive progenitor cells and demonstrated a long lasting contribution of the infused marrow to hematopoiesis. The gene marking technique is now being used to evaluate the efficacy of purging and the impact of growth factor treatment on long and short term hematopoietic reconstitution.