Generation and characterization of conditional PU.1 knock-out mice. (A) Generation of a conditional targeted allele of the murine PU.1 gene. Cre recombination sites (loxP, black arrowheads) were inserted distal to the SpeI site in intron 3 and 435 base pair (bp) distal to the end of exon 5 as indicated. Shown are the predicted structures of the wild-type allele (WT), the targeted allele with the loxP sites (loxP), and the allele after Cre-mediated recombination, in which exons 4 and 5 have been deleted (KO). The relative location of the probe used in Southern blot analysis (a 1.3-kb BamHI/EcoRI fragment) is also shown. (B) Southern blot analysis of Mx1-Cre × PU.1^F/F mice 2 weeks following pI-pC injection. BM indicates bone marrow. (C) Morphology of adult bone marrow cells in PU.1^F/F (left) or Mx1-Cre × PU.1^F/F (right) mice 3 weeks after injection of pI-pC (May-Giemsa staining; top panels: 400×; bottom panels: 1000×).