Addition of phosphate to proteins by kinases, or its removal by phosph
atases, is probably the control mechanism most often used by cells to
maintain homeostasis. This mechanism presents the advantage of being f
ast, versatile, and easily reversible. It is used by all organisms fro
m bacteria to man. Although more is known about the kinases, recent st
udies are beginning to shed light on the role of phosphatases, the enz
ymes that are responsible for terminating the effects of phosphorylati
on. These enzymes are perfect candidates for controlling all the cruci
al check points during cell cycle traverse, and as such, will be found
to be responsible for many important decision in the life of a cell,
including entry into replicative senescence.