Imatinib inhibits osteoblast proliferation but does not induce apoptosis. Osteoblast cultures were stained with the fluorescent dye CFSE, cultured for a period of 7 days in the presence (□) or absence (■) of imatinib (3 μM), then analyzed by flow cytometry. (A) Imatinib reduced the number of cell divisions compared with the untreated control. (B) To determine whether imatinib induced apoptosis, the effect of imatinib on cell cycling was examined using propidium iodide (PI). No difference in cell cycling was observed between untreated and imatinib treated cultures, with most cells accumulating in G₁ phase of the cell cycle.