Roger Kornberg, at Stanford University, determines in 2000, after more than a decade of extraordinarily demanding technical work, the detailed three-dimensional structure of RNA polymerase II caught in the act of transcription — the enzyme responsible for reading the DNA of eukaryotic cells (with a nucleus, including all human cells) and producing from it the corresponding messenger RNA molecule. The structure, obtained via X-ray crystallography of a massive molecular complex of twelve distinct protein subunits, reveals with atomic precision how the enzyme slides along the DNA double helix, temporarily separates the two strands, and synthesizes the complementary RNA strand by reading one of them as a template. Kornberg's work completes, in the specific context of eukaryotic cells, what the Jacob and Monod operon model had conceptually established for bacteria and what Khorana, Nirenberg, and Holley had established for the genetic code: the detailed set of molecular mechanisms by which a cell converts information stored in DNA into functional molecules. The structure also reveals the exact points where various regulatory proteins bind that activate or repress specific genes, knowledge fundamental to understanding diseases caused by failures in transcriptional regulation, including numerous types of cancer.