In any multi-cellular organism, the balance between cell division and cell death maintains a constant cell number. Both cell division cycle and cell death are highly regulated events. Whether the cell will proceed through the cycle or not, depends upon whether the conditions required at the checkpoints during the cycle are fulfilled. In higher eucaryotic cells, such as mammalian cells, signals that arrest the cycle usually act at a G1 checkpoint. Cells that pass this restriction point are committed to complete the cycle. Regulation of the G1 phase of the cell cycle is extremely complex and involves many different families of proteins such as retinoblastoma family, cyclin dependent kinases, cyclins, and cyclin kinase inhibitors.Key words: Cell cycle, cyclin dependent kinase, cyclin, cyclin kinase inhibitor.
Retinoblastoma family of proteinsThe retinoblastoma family consists of three members: pRb, p107, and p130, that are all important in the cell cycle regulation. Studies from knockout mice showed that pRb null mutation is embryonically lethal, while p107 or p130 knockout mice have no known * Corresponding author.Tel: (301) 517-0355; Fax: (301) 517-0344; E-mail: scottd@usa.redcross.org Abbreviations used in this paper: pRb,retinoblastoma protein; CDK, cyclin dependent kinase; CKI, cyclin kinase inhibitor; IgM, immunoglobulin M; BCR, B cell receptor; BHFR, dihydrofolate reductase; TGF-β, transforming growyh factorβ.