To find a biological answer to the question “Who are we?” we might look to the human genome. Certainly, when the Human Genome Project first produced a draft of the 3 billion-base-pair sequence, it was touted as a blueprint for human life. Less than a decade later, however, most experts recognize that our genomes capture only a part of who we are. Researchers have become aware, for example, of the influence of epigenetic phenomena — imprinting, maternal effects, and gene silencing, among others — in determining how genetic material is ultimately expressed. Now comes the notion that the genomes of microbes within us must also be considered. Our bodies are, after all, composites of human and bacterial cells, with microbes together contributing at least 1,000 times more genes to the whole.