HOW VIRAL ONCOGENES MAKE THE CELL-CYCLE

Oncogenic viruses provide their host cells with additional growth stim uli, thereby extending their proliferative capacity. This implies that viral oncogenes can override growth-suppressive signals, which contro l cell-cycle progression in untransformed cells. Viral oncoproteins de regulate cell-cycle control by interfering with receptor-mediated sign al transduction pathways and the function of nuclear cell-cycle regula tory proteins. As a consequence of these regulatory interactions, many viral oncogenes induce the expression of cellular genes required for cell-cycle progression, including genes encoding G1 cyclins. Apparentl y, different oncogenic viruses target different subsets of these cell- cycle regulatory pathways to transform cells.