Cell Cycle and Cervical Carcinoma 2-3, and those cases of CIN 1 associated with high-risk HPV types. Kelley et al., however, reported that no altera- tions in p16 expression were detected in cervical cell lines, including 8 HPV-positive cell lines, 2 HPV-negative cell lines containing mutant Rb, and one tumorigenic cell line derived from nor- mal cervical cells following transfection with HPV-16 and v-H-ras (CX16-2HR).14 These results confirm that mutational inactivation of p16INK4 is a rare event in tumor samples with compromised Rb activity. Volgareva et al. reported that all stages of CINs and carcinomas analyzed were heterogeneous with respect to p16 expression. Samples that showed expression of p16 in 25% of cells or more were identified in a side-by-side study with samples that stained poorly or lacked staining. These samples are evidence that p16- negative cervical neoplasms and carcinomas do exist.18 The molecular basis for overexpression of p16 in cervical squamous carcinoma remains unclear. Some explanations include a genetic mutation or a supragenetic event at the protein level, such as increased half-life of the p16 protein. The interac- tion of p16 with pRb is thought to be central to the role of p16 in controlling cell cycle progres- sion. It has been suggested that HPV infection leads to HPV-E7 binding to pRb, which in turn results in increased p16 expression. Some studies have suggested that this process only occurs in cases of infection by high-risk types of HPV: p16 transcription may also be directly induced by the transcription factor E2F released from pRb after binding of the viral oncoprotein E7.19 p16 has been used as a biomarker for dyspla- sia in the diagnosis of cervical squamous lesions and has the potential to be used as an additional screening tool.8,20,21 The overexpression of p16 is closely associated with high-risk HPV infection and high-grade CIN.22 A recent report showed that in cervical biopsy specimens, the staining pattern of p16 and a high percentage of p16-posi- tive cells are closely related to infection with high-risk HPV types 16 and 18, and with CIN 2/3. Several studies of p16 in liquid-based cytology specimens suggested that p16 could be used in cervical screening as a marker for persistent high-risk HPV infection, and HSIL could also be useful in resolving ambiguous cases involving a differential diagnosis of cervical neoplasia (Table 1).23,24 Cyclin D1 The cyclin D1 (PRAD-1, CCND-1) gene maps to 11q13 and shows the characteristics of a cellular oncogene. Expression of cyclin D1 moderately oscillates throughout the cell-cycle, reaching peak levels in G-phase.7 Cyclin D1 serves as a key sen- sor and integrator of extracellular signals in early to mid-G1 phase, mediating its function through binding the CDKs, histone acetylase, and histone deacetylases to modulate local chromatin struc- ture around the genes that are involved in regula- tion of cell proliferation and differentiation.25 In addition to CDK-binding functions, a body of evidence now indicates that D-type cyclins have CDK-independent properties. These properties are important for cellular growth, metabolism, and cellular differentiation. Cyclin D1 forms physical associations with more than 30 transcription fac- tors and transcriptional co-regulators.25 Several nuclear receptors, including the androgen receptor (AR), estrogen receptor (ER), thyroid hormone receptor, and peroxisome proliferator-activated receptor (PPAR), bind directly to cyclin D1 within cultured cells.26,27 Both basal and ligand-depen- dent transactivation of nuclear receptors is regulated by cyclin D1. Genetic aberrations in the regulatory circuits that govern transit through the G1 phase of the cell cycle occur frequently in human cancer, and overexpression of cyclin D1 is one of the most commonly observed alterations.25 One model sug- gests that the overexpression of cyclin D1 may serve as a driving force through its cell-cycle regulating function. Cyclin D1 is amplified and/or overexpressed in a substantial proportion of different human tumors. The role of cyclin D1 in cervical carcinogenesis is not clearly understood, and controversial results have been described. Cho et al. found that cyclin D1 levels were significantly lower in HPV-positive HSIL, invasive SCC, or adenocarcinoma compared to HPV-negative cases and normal cervical epithe- lium, consistent with other authors' results.28-30 Contrary to this, the results of Nichols et al. described elevated cyclin D1 mRNA levels in Yonsei Med J Vol. 46, No. 5, 2005