Restriction beyond the restriction point: mitogen requirement for G(2 )passage

Cell proliferation is dependent on mitogenic signalling. When absent, normal cells cannot pass the G(1 )restriction point, resulting in cell cycle arrest. Passage through the G(1 )restriction point involves inactivation of the retinoblastoma protein family. Consequently, loss of the retinoblastoma protein family leads to loss of the G(1 )restriction point. Recent work in our lab has revealed that cells possess yet another mechanism that restricts proliferation in the absence of mitogens: arrest in the G(2 )phase of the cell cycle. Here, we discuss the similarities and differences between these restriction points and the roles of cyclin-dependent kinase inhibitors (CKIs) herein.