Cell proliferation assays monitor actively dividing cells, expressed either as the actual number of active cells or the ratio of proliferating to non-proliferating cells in culture [25–27]. Quiescent cells that are healthy but not proliferating are not detected by cell proliferation assays. Measurement of DNA synthesis has been used as a specific marker in actively dividing cells. In these assays, labeled nucleotide analogs (e.g., [3H]-thymidine or 5-bromo-2′-deoxyuridine [BrdU]) are added to cells and are incorporated into the replicated DNA during the S phase of the cell cycle. The amount of labeled nucleotide is then quantified by: (i) measuring the total amount of labeled DNA in the cell population, or by (ii) counting the number of labeled nuclei under a microscope. The growth rate of the cells will determine the rate of cell turnover which dictates the incubation period with the labeled nucleotide analogs. Quantitation of DNA content of cells based on radioisotopic assays are terminal, labor-intensive and present handling, storage, and disposal issues. They are also time-consuming and subject to operator errors especially for medium to high-throughput applications [25]. The clonogenic assay measures the effect of a test compound on the proliferating fraction of the population (for a review of these assays see references)