In 2004 genetic testing revealed that Masha Gessen had a mutation that predisposed her to ovarian and breast cancer. 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, pointing the way towards the radical transformation genetic information is engineering in our sense of who we are and what we might become.