The multistage process of carcinogenesis involves the progressive acquisition of mutations, and epigenetic abnormalities in the expression, of multiple genes that have highly diverse functions. An important group of these genes are involved in cell cycle control. Thus, cyclin D1 is frequently overexpressed in a varety of human cancers. Cylin D1 plays a critical role in carcinogenesis because (i) overexpression enhances cell transformation and tumorigenesis, and enhances the amplification of other genes, and (ii) an antisense cyclin D1 cDNA reverts the malignant phenotype of carcinoma cells. Therefore, cyclin D1 may be a useful biomarker in molecular epidemiology studies, and inhibitors of its function may be useful in both cancer chemoprevention and therapy. We discovered a paradoxical increase in the cell cycle inhibitors protein p27(Kip1) in a subset of human cancers, and obtained evidence for homeostatic feedback loops between cyclins D1 or E and p27(Kip1). Furthermore, derivatives of HT29 colon cancer cells with increased levels of p27(Kip1) showed increased sensitivity to induction of differentiation. This may explain why decreased p27(Kip1) in a subset of human cancers is associated with a high grade (poorly differentiated) histology and poor prognosis. Agents that increase cellular levels of p27(Kip1) may, therefore, also be useful in cancer therapy. Using an antisense Rb oligonucleotide we obtained evidence that the paradoxical increase in pRb often seen in human colon cancers protects these cells from growth inhibition and apopotosis. On the basis of these, and other findings, we hypothesize that homeostatic feedback mechanisms play a critical role in multistage carcinogenesis. Furthermore, because of their bizarre circuitry, cancer cells suffer from 'gene addiction' and 'gene hypersensitivity' disorders that might be exploited in both cancer prevention and chemotherapy.