Young Tae Kim and Min Zhao The development and progression of cervical carcinoma has been shown to be dependent on various cellular genetic and epigenetic events, especially alterations of the cell cycle machinery at various checkpoints. The precise control of the cell cycle in mammalian cells is regulated by the acti- vity of cyclin-dependent kinases (CDK1, CDK2, CDK4, CDK6) and their essential activating coen- zymes, the cyclins (cyclins A, B, D, E). The kinase activities of these CDKs are regulated by the abundance of their partner cyclins, phosphor- ylation by various kinases, de-phosphorylation by cell cycle phosphatases, and interaction with CDK-inhibitory proteins (CDKIs).3-5 The CDK family is an important group of mole- cules that regulate cell proliferation. In addition, two classes of mammalian cyclin-dependent kinase inhibitors (CDKIs) have been described: the CIP/KIP family, comprised of p21, p27, and p57, and the INK4 family, comprised of p15, p16, p18, and p19.6 The INK4 molecules specifically inhibit cyclin D complexes by interaction with the CDK4 and CDK6 components. The KIP family is pro- miscuous, affecting cyclin E, cyclin A/CDK2, and cyclin B/CDK1 by binding both the cyclin and CDK subunit.3 In this article, we review the current knowledge of the role of cell-cycle regulatory proteins and their alterations in cervical cancer. CDKs, cyclins, and CDKIs generally function within several defined pathways, including the p16INK4A-cyclin D1-CDK4/6-pRb-E2F pathway, the p21WAF1-p27KIP1 -cyclinE-CDK2 pathway, and the p14ARF-MDM2- p53 pathway.7 Each of these components plays either a positive or a negative role in cell- cycle control mechanisms in cervical carcinogenesis. Alterations in CDKs, CDKIs, and cyclins can lead to uncontrolled proliferation and might contribute to malignant transformation of the uterine cervix. The p16INK4A -cyclin D1-CDK4/ 6- pRb-E2F pathway p16INK4A The p16INK4A gene maps to 9p21, contains three exons, and encodes a nuclear phosphoprotein with a molecular weight of 16 kDa. The p16 pro- tein functions in the negative regulation of the cell cycle through the inhibition of cyclin- dependent kinases 4 and 6 and interactions with cyclin D1.8 Yonsei Med J Vol. 46, No. 5, 2005 In the absence of p16, CDKs bind to cyclin D1, and the Retinoblastoma protein (pRb) is phos- phorylated. Phosphorylation of pRb leads to its deregulation at the G1/S checkpoint, and cell proliferation is switched on. In a variety of human malignant tumors and cell lines, the p16INK4A gene is inactivated by various genetic mechanisms, including point mutations, homozygous deletions, and hypermethylation of CpG islands in the p16INK4A promoter. Kim et al. found a high per- centage of p16 exon 2 mutations in cervical cancer specimens.9 Dong et al. and Nuovo et al. also described hypermethylation of the p16 promoter, documenting inactivation of the gene as a fre- quent epigenetic event in cervical carcinoma.10,11 However, mutations in p16 in these lesions have not been confirmed.10 In some reports, mutation and deletion of p16 are suggested to be rare events: no gene alterations were detected in most studies of primary cervical carcinomas or cervical cancer cell lines, indicating that they are not required for the development of this tumor type. These data are concordant with those of previous reports.12-14 Conflicting results about the expression of p16 protein have been obtained from studies of the role of p16 in the pathogenesis of cervical cancers.9,13,15,16 Overexpression of the p16 protein is a characteristic of displastic and neoplastic alterations of the cervical epithelium. The portion of p16-positive samples increases as the tumor progresses from the CIN I to the invasive carci- noma stage.16 Some reports have shown that p16 expression is detectable by immunohistochemistry in cervical neoplasia and that this expression may be a direct result of HPV infection with resultant inactivation of pRb, which is known to bind p16.15,16 Squamous cell carcinomas (SCCs), high- grade squamous intraepithelial lesions (HSILs), and adenocarcinomas (ACs) of the cervix have shown increased p16 immuno-staining.17 Klaes et al. examined the utility of p16 immunostaining to assess inter-observer agreement in the diagnosis of cervical intraepithelial neoplasia (CIN). In that study, 194 cervical cone biopsies were examined and stained with an antibody to p16. A significant increase in inter-observer agreement when ex- amining p16 expression was observed. The p16 expression was restricted to cervical cancer, CIN