One brain, many genomes

We each begin life as a single cell harboring a single genome, which—over the course of development—gives rise to the trillions of cells that make up the body. From skin cells to heart cells to neurons of the brain, each bears a copy of the original cell's genome. But as anyone who has used a copy machine or played the childhood game of “telephone” knows, copies are never perfect. Every cell in an individual actually has a unique genome, an imperfect copy of its cellular ancestor differentiated by inevitable somatic mutations arising from errors in DNA replication and other mutagenic forces ( 1 ). Somatic mutation is the fundamental process leading to all genetic diseases, including cancer; every inherited genetic disease also has its origins in such mutation events that occurred in an ancestor's germline cells. Yet how many and what kinds of somatic mutations accumulate in our cells as we develop and age has long been unknown and a blind spot in our understanding of the origins of genetic disease.