Structure of the HOXB4 and control neoretroviruses and the experimental outline. (A) Diagrammatic representation of the integrated HOXB4 and neoproviruses. Expected size of the full-length viral transcripts and also those initiated from the PGK promoter are shown, as are the sites for the various restriction enzymes used in this study. (B) Experimental outline showing the number of HOXB4 and neo mice from 3 transplantation experiments that were used in this study and the time posttransplantation when they were analyzed. Also shown is the initial gene transfer to the transplanted bone marrow inoculum received byneo and HOXB4 mice in these transplantations. (C) Southern blot analysis of DNA isolated from bone marrow and thymus of some of the neo (killed 32 weeks posttransplantation) andHOXB4 mice (killed 32, 41, and 52 weeks posttransplantation) used in this study to demonstrate the presence of the integrated provirus. DNA was cut with Kpn I, which releases theneo (2.7 kb) and the HOXB4 (3.9 kb) proviruses, and the blot was successively hybridized to probes specific for the neoand HOXB4 genes (full-length HOXB4 cDNA was used as a probe). The endogenous murine HOXB4 is detected at 1.3 kb by the HOXB4 probe and provides a single gene copy control of loading. In some of the HOXB4 mice, in addition to the full-length HOXB4 provirus, a weaker 2.7-kb proviral signal is detected with the neo probe but not with the HOXB4probe. Because this fragment failed to hybridize to the HOXB4probe, it likely represents a rearrangement resulting in the loss of the HOXB4 gene from some of the integrated proviruses. Kp,Kpn I; E, EcoRI; B, BamHI; SD, splice donor; SA, splice acceptor; CFC, colony-forming cells; B, bone marrow; T, thymus.