In the absence of growth factors, the retinoblastoma (RB) protein binds a transcription activator called E2F, preventing the cell from entering S stage. A mutation in E2F occurs that causes RB to no longer bind it. This mutant E2F would be considered a(n) ________.

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E2F promotes entry into S stage and would be considered a proto-oncogene. The mutant E2F promotes the cell cycle regardless of feedback from RB, and would thus be considered an oncogene.

In the Absence of Growth Factors, the Retinoblastoma (RB) Protein