Understanding of cyclin-dependent kinase (CDK) regulation in mammalian
cells has deepened even as the functions ascribed to these enzymes ha
ve multiplied. We know from crystallographic studies how a prototypic
CDK-cyclin complex is activated and inactivated; the challenge now is
to extend this knowledge to other CDKs involved in cell cycle progress
ion. At the same time, as CDKs turn up in some unexpected places, inte
rest in CDK regulation has spread beyond the cell cycle field.