Transient overexpression of p21 induces macrophage differentiation of U937 cells. Shown is an immunofluorescent double staining of a transfected cell. Exponentially growing U937 cells were cotransfected with an empty CMV expression vector (top panel) or a CMV vector containing p21 cDNA (bottom panel) together with a CMV plasmid expressing Green Lattern Protein, a modified version of GFP. Green fluorescence was used as a marker for transfected cells (A and C). CD11b expression was detected using a Texas red-conjugated antimouse secondary antibody (B and D). Note the morphological change evident in the CMV-p21-transfected cell (C) compared with the CMV(−) cell (A).