For this reason, it is likely that the members of the E2F family are important regulators of oncogenic transformation. The E2F3-transduced lines exhibited a normal growth rate during the first 48 hours of culture, but then proliferated at twice the rate of the control cells until reaching confluency after only 72 hours. Unlike the E2F1�C3 lines, cells transduced with E2F4, E2F5 and E2F6 lagged behind the control MINR1 3T3 line, exhibiting little or no cell growth over the first 48 hours of culture. These lines underwent approximately two doublings during the next 24 hours, but arrested at roughly 72 hours, before reaching 100% confluence. These data suggest that deregulated expression of E2F4, E2F5 and E2F6 can slow cell cycle progression and render cells more susceptible to contact inhibition. E2F gene expression is normally regulated by signals from growth factor receptors, and is tightly coordinated with the cell cycle. To determine the effect of forced expression of individual E2F family members on cell growth in the relative absence of growth factors, we plated each E2F-transduced line at medium density in low serum medium, and monitored E2F family member expression and cell number over a two day culture period. The MINR1 empty vector line exhibited very low expression of endogenous E2F family members following serum withdrawal, while the E2F3�C6 transductants exhibited efficient, serum-independent expression of E2F3, E2F4, E2F5 and E2F6, respectively. Serum deprivation actually induced accumulation of transgenic E2F1 and E2F2 protein in the E2F1and E2F2-transduced lines, suggesting that growth factor-coupled mechanisms that limit E2F1 and E2F2 protein expression at a post-translational level may be operative during growth in high serum in our system. During the first 16 hours of serum deprivation, control-transduced 3T3 cultures increased in cell number by approximately 50%, but fell precipitously by 30 hours. These results, along with the initial drop in cell number at 8 hours, suggest a relatively rapid conversion from cell growth to cell death in these cultures upon serum withdrawal. The growth of most cells, including fibroblasts, requires integrinmediated signals provided through attachment to a solid matrix.