In our studies, retroviral Phellodendrine expression of E2F2 and E2F3 promoted both serum- and contact-independent growth of normal fibroblasts, Catharanthine-hemitartrate consistent with previous in vitro studies in both transient and stable over-expression systems. These data are also consistent with in vivo studies in which targeted expression of E2F2 or E2F3 in epithelial tissue led to epithelial hyperplasia, and in the case of deregulated E2F2 expression, led to cortical thymoma formation. In our studies, E2F3a exhibited stronger transformation activity than E2F2. This may result from the more stable expression of transgenic E2F3a protein in this system as compared to E2F2, and suggests that E2F3a and E2F2 may be differentially subject to post-translation control mechanisms. These activities may together account for strongest proliferative capacity of the E2F3a-transgenic fibroblasts in our studies. E2F3b, a splice variant of E2F3 that contains coding regions unique from E2F3a, was not tested in these studies. This family member might be expected to be neutral or anti-oncogenic, as E2F3b has been shown to preferentially bind pRb and repress S-phase genes in fibroblasts in vitro, but further studies will be required to address the oncogenic capacity of this E2F family member. Forced expression of E2F4 and E2F5 negatively impacted fibroblast growth in our experiments, consistent with their defined roles in enforcing G1 arrest. E2F4 and E2F5 can exhibit oncogenic activity, but only when expressed together with other oncogenes such an activated mutant of Ras. The empty MSCV retroviral vector in our studies exhibited measurable transforming activity in 3T3 fibroblasts, and this was abrogated by E2F4 and E2F5. These results suggest that these E2F family members can also have anti-oncogenic or tumor suppressive activity. Unlike E2F1�C3, E2F4 and E2F5 are highly expressed in quiescent cells, lack a cyclin A-binding domain, and associate with p107 and p130 instead of pRB. These factors also lack nuclear localization domains, and depend upon their association with pocket proteins for nuclear translocation.