Cayman's XTT Cell Proliferation Assay provides an easy to use tool for studying induction and inhibition of cell proliferation in any in vitro model. The assay is based on the extracellular reduction of XTT by NADH produced in the mitochondria via trans-plasma membrane electron transport and an electron mediator. Reduction of XTT produces a water-soluble formazan that dissolves directly into the culture medium, eliminating the need for an additional solubilization step. This kit will allow investigators to screen drug candidates involved in cell cycle regulation.