PU.1 disruption at the CMP or GMP stage. (A) PCR analysis of flanked PU.1 alleles in control and Cre-transduced GMPs. The flanked PU.1 allele became undetectable after the Cre transduction into PU.1^F/F GMPs. M indicates the molecular weight marker. (B) RT-PCR analyses of cytokine receptors in PU.1^Δ/Δ CMPs. M indicates the molecular weight marker; P, the positive control. (C) Myeloid colony assays of single sorted PU.1^F/F and PU.1^Δ/Δ CMPs/GMPs in the presence of SCF, GM-CSF, IL-3, Epo, and Tpo. PU.1^Δ/Δ CMPs and GMPs could not form mature M, G, or GM colonies, but formed myeloblast colonies. BL indicates blast. (D) PU.1^Δ/Δ GMPs formed colonies composed mainly of immature myeloid cells (May-Giemsa staining, 600×), which expressed only low levels of CD11b.