Effect of p18 and p27 on proliferation of differentiated cells.

We cultured p18 and p27 MEL cell transfectants in the presence of 5 mmol/L HMBA for 12 hours. HMBA was washed out, and the transfectants were cultured in the presence of 1 μg/mL Dox for 12 hours. The cells were plated in plasma clots and incubated for 4 days (solid bars) and 7 days (open bars) at 37°C in the absence of both HMBA and Dox. The clots were then stained with benzidine and hematoxylin as described in “Materials and methods.” The proliferative capacity of cells that were committed to differentiation was determined by counting the number of cells in at least 100 colonies that stained positive with benzidine.