Southern blot analysis of proviral integration in secondary transplant recipients after MSCV-m91Neo transduction of X-CGD bone marrow. Genomic DNA was extracted from bone marrow obtained from an X-CGD control mouse, an X-CGD mouse used as a donor for secondary transplants 11 months posttransplantation with MSCV-m91–transduced BM, and three secondary transplant recipients (17-25, 17-26, and 17-27). Ten micrograms of genomic DNA was digested with EcoRI to generate junctional fragments containing provirus and adjacent genomic DNA. After agarose gel electrophoresis, Southern blots were prepared and probed with radiolabeled neomycin phosphotransferase cDNA. The band derived from an endogenous Neo gene in the X-CGD mice is marked by an arrow on the left. Open circles indicate a junctional fragment present in the donor that was also seen in all three recipient mice. Other junctional fragments were seen only in some or all of recipient mice (indicated by the other symbols).