Immature myeloid progenitor cells with biphenotypic differentiation potential can be immortalized by infection with replication-defective MSCV. (A) Schematic representation of the immortalization procedure. Horse serum was included in the cultures along with the indicated growth factors after infection. (B) Wright-Giemsa staining of uninfected (left panel) and infected (right panel) bone marrow (BM) cells after 1 month in culture. Original magnification × 400. (C) Wright-Giemsa staining of 2 immortalized cell lines (BM-7 and BM-39) before and after treatments with G-CSF (3 days), GM-CSF (5 days), or PMA (2 days). BM-7 harbors a single MSCV Evi1 integration, while BM-39 harbors a Prdm16 integration. Both cell lines respond similarly to cytokine stimulation. Original magnification × 400. Images were obtained with the use of an Olympus Vanox AHBS3 microscope and a Nikon DXM1200F digital camera. Images were processed using Image-Pro Plus 5.1.