Macrophage development can be restored by PU.1 transduction of PU.1-deficient 503 cells. (A) PU.1 transduction of 503 cells (503-PU) restored the development of cells with typical macrophage appearance as shown by Wright-Giemsa staining. (B) Immuno-cytochemistry was used to show that 503-PU cells expressed the macrophage marker F4/80, unlike the parental cells (503). Positive-staining cells demonstrate an orange-brown reaction product. Note the presence of F4/80⁺ 503-PU cells with lobate nuclear morphology of monocytes. (C) An adherent subpopulation 503-PU cells was also positive for the macrophage subset marker sialoadhesin by immunocytochemical staining, whereas no positive cells were found in parental 503 cultures.