For $150, you can buy a CRISPR kit online and use it to engineer heartier gut bacteria in your kitchen. That's thrilling, but the technology is giving Jennifer Doudna, an inventor of the gene-editing method, nightmares. Easy genetic modification could mean cures for cancer (yay!), kitty-sized pigs (squee!), and, yes, designer babies (ack). In her new book, A Crack in Creation, Doudna urges innovators to slow their roll. Here she considers the daunting prospects and promises of the monster-maker she created.