In 2004 genetic testing revealed that Masha Gessen had a mutation that predisposed her to ovarian and breast cancer. The discovery initiated Gessen into a club of sorts: the small (but exponentially expanding) group of people in possession of a new and different way of knowing themselves through what is inscribed in the strands of their DNA. As she wrestled with a wrenching personal decision—what to do with such knowledge—Gessen explored the landscape of this brave new world, speaking with others like her and with experts including medical researchers, historians, and religious thinkers. Blood Matters is a much-needed field guide to this unfamiliar and unsettling territory. It explores the way genetic information is shaping the decisions we make, not only about our physical and emotional health but about whom we marry, the children we bear, even the personality traits we long to have. And it helps us come to terms with the radical transformation that genetic information is engineering in our most basic sense of who we are and what we might become.