We have concentrated on how SCE is measured, how it correlates with other measures of cellular damage, and how SCE can be used as a tool to study cell cycle kinetics. We have not discussed the biological significance of SCEs, i.e. why they occur spontaneously, and what role, if any, in cellular preservation they play. Similarly, our discussion of the mechanisms of formation of SCE has been deliberately brief. These are important considerations, but the answers to them are still largely only speculative. What is clear is that SCE is an extremely interesting biological phenomenon from a variety of points of view, and even though the mechanisms underlying SCE are not yet well understood, measurement of these events has produced much useful and provocative information.