Our goal is to gain a better understanding of the control of cell cycle progression in normal cells; and then to utilize the information to better understand the unregulated growth of tumors. This laboratory has for many years been a leader in the development of novel cytometric techniques and their use in the study of cell proliferation. Recently, we have utilized this approach to demonstrate that cyclin D1 levels vary dramatically through the cell cycle, with low levels during S phase required for DNA synthesis, and stimulation of these levels during G2 phase necessary for continued cell cycle progression. At the same time, p27Kip1 levels determine the rate of passage through G1 phase, and the rate of cell cycle progression. Importantly, the expression patterns of these proteins seen in normal cells are dramatically altered in many tumor cells.