Because I find it hard to relate to something as small as the structure of the human genome, I like to imagine it scaled up a millionfold. At this size, each DNA molecule—a chromosome—is as wide as a ramen noodle. Laid end to end, all 46 of the scaled-up chromosomes that compose a cell’s genome would stretch from New York to Kansas City, although they instead fold up to fit inside a structure the size of a house—the cell nucleus. Collectively, the 46 chromosomes contain two sets of roughly 20,000 genes. Each gene spells out a coded message telling the cell how to make a particular protein; at the millionfold scale, a gene is as long as a car.