This paper describes studies developing a method to measure the absolute concentration of a key metabolic enzyme in living cells. The method allows scientists to not determine the absolute concentration of that enzyme in living cells and, therefore, to determine the health of cells in living tissues and to distinguish different cells types (with different metabolic profiles) from each other in living tissues. The technology is a further development of methods to study stem cells in living tissues in order to study their biology, to determine the health of living tissues, to distinguish normal cells from abnormal (perhaps cancerous) cells and to help define the risks to normal cells in adults and embryos of environmental chemicals.