Complexes between the retinoblastoma protein (pRb) and the transcription factor E2F-1 are thought to be important for regulating cell proliferation. We have shown previously that the E7 oncoprotein from human papillomavirus type 16, dependent upon its binding to pRb proteins, induces proliferation, disrupts differentiation, and induces apoptosis when expressed in the differentiating, or fiber, cells of the ocular lenses in transgenic mice. Mice that carry a null mutation in E2F-1 do not exhibit any defects in proliferation and differentiation in the lens. By examining the lens phenotype in mice that express E7 on an E2F-1 null background, we now show genetic evidence that E7's ability to alter the fate of fiber cells is partially dependent on E2F-1. On the other hand, E2F-1 status does not affect E7-induced proliferation in the undifferentiated lens epithelium. These data provide genetic evidence that E2F-1, while dispensible for normal fiber cell differentiation, is one mediator of E7's activity in vivo and that the requirement for E2F-1 is context dependent. These data suggest that an important role for pRb-E2F-1 complex during fiber cell differentiation is to negatively regulate cell cycle progression, thereby allowing completion of the differentiation program to occur.