Neoplastic diseases are characterized by uncoordinated cell growth. Cellular proliferation follows an orderly progression through the cell cycle, which is governed by protein complexes composed of cyclins and cyclin-dependent kinases. These complexes exert their regulatory function by phosphorylation of key proteins involved in cell cycle transitions, such as the product encoded by the retinoblastoma gene (pRB). Mutations and overexpression of cyclins and cyclin-dependent kinases, mainly cyclin D1 and Cdk4, have been reported and proposed to be oncogenic events. More recently, a new family of negative regulators functioning as Cdk-inhibitory molecules has been identified. Because of their recessive nature in cell cycle control and the fact that some of them are mutated in human tumors, it has been suggested that they may also function as tumor suppressor genes. It appears that the molecular networking of these proteins and complexes impact on two fundamental cell cycle regulators: p53 and pRB. Cross-talk pathways between these two nuclear proteins are being delineated, implying potential links between p53 and pRB in cell cycle control, apoptosis, and tumor progression. In addition, the high rate and mutation pattern of TP53 and RB in primary tumors have rendered them prototype tumor suppressor genes. Furthermore, detection of TP53 and RB mutations and altered expression of their encoded products appear to be of clinical significance, often correlating with prognosis, when identified in specific cancers. Based on these findings, new strategies are being developed in the emerging field of gene replacement-therapy.