... that a previously unrecognised balance between the cyclin-dependent kinase (CDK) inhibitor p21 and the checkpoint kinase Chk1 controls cyclin D–CDK activity during G2 arrest. In non-transformed cells, p21 activates RB in G2 by inhibiting cyclin D1 complexed with CDK2 or CDK4. The resulting G2 exit, which precedes...

...). * Author for correspondence (e-mail: [email protected] ) 16 2 2002 © The Company of Biologists Limited 2002 2002 C2 domain HECT domain G2 arrest Ubiquitin ligase The ubiquitin-proteasome system controls crucial steps in a wide variety of biological processes...