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RNA polymerase II, or pol II, as the enzyme is called, is the heart of the machinery that controls everything that cells do-from differentiating into all the tissues of a developing embryo to responding to everyday stresses. Now, in two papers published online today by Science (www.sciencexpress.org), cell biologists can get their best look yet at just how the pol II enzyme of yeast and, by implication, of other higher organisms performs its critical role. The papers describe the atomic structure of yeast pol II by itself, and, at slightly lower resolution, the enzyme caught in the act of DNA synthesis. The structures open the door to seeing exactly how the enzyme interacts with the many other proteins that regulate its activity.