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Enucleation of L cells leads to loss of the capacity to produce interferon, showing that the cell nucleus is essential for interferon formation. However, when the cells were enucleated while interferon formation was proceeding, the cytoplasts were capable of continuing to synthesize interferon by a process shown to be protein synthesis, showing that the interferon messenger RNA leaves the nucleus after synthesis. Reconstructed cells were obtained by Sendai virus fusion of karyoplasts and cytoplasts. Such reconstructed cells were capable of producing at least as much interferon (43 interferon units/104 nucleated cells) as control cells (31 interferon units/104 nucleated cells).