Biochemists have an insatiable demand for analytical sensitivity. This is because some of the most interesting biological materials are present in tissues at very low concentrations and some crucial biochemical questions concern amounts of materials in single cells or parts of cells. In many (perhaps most) cases, the needed sensitivity can be provided by methods based on radioactive labeling. There are, nevertheless, other situations in which tracer methods are not sensitive enough, are difficult to apply, or a direct chemical assay would simply take less time. For example, it might be difficult to measure the concentration of glucose-6-P in a microgram of tissue with radioactive methods. With enzymatic cycling, this is no problem.