Monocytic precursors are found in the PU.1 null liver. Cells from normal and PU.1−/− neonate livers were triple stained with Moma-2^FITC and either unconjugated ER-MP12 (MP12) or ER-MP20 (MP20) monoclonal antibodies, along with F4/80^PE, CD11b^PE, or Gr-1^PE-conjugated antibodies. MP12 or MP20 were stained with a biotinylated isotype-specific (IgG2a) secondary antibody followed by Streptavidin Cy-Chrome (A) Moma-2 versus MP12 or MP20 FACS data of normal and PU.1−/− liver cells demonstrate single positive, double positive, and unstained cells. The quadrants were determined based on background staining of cells with isotype control antibodies. The percentage of each positive staining population is indicated in the corners of the quadrants. (B) Histogram plots of F4/80, Mac-1, or Gr-1 cells found in gated normal and PU.1−/− MP12⁺Moma-2⁺ population. The rectangular box in (A) was the gate used to analyze the double positive populations.