Five years ago, Daniel MacArthur set out to build a massive library of human gene sequences—one of the biggest ever. The 60,706 raw sequences, collected from colleagues all over the globe, took up a petabyte of memory. It was the kind of flashy, blockbuster project that would secure MacArthur a coveted spot in one of science’s top three journals, launching his new lab at the Broad Institute into the scientific spotlight.