Imagine, for a moment, the entire human genome as a vast library. Its books are written in an alphabet containing just four letters: A, C, G, and T, the four building-block chemicals of DNA. The human genome has more than 3 billion such letters—6 billion per cell if you count the genomes of both parents. Reframed as a library of books, with about 250 words per page and 300 pages per book, we might think of ourselves—or rather the instructions to build, maintain, and repair ourselves—as written in about 80,000 books.
