Jennifer Doudna tells the story of how studying the way bacteria fight viral infection turned into a genomic engineering technology that has transformed molecular biology research. In 2013, Doudna and her colleagues developed the CRISPR-Cas9 gene expression system that, when introduced into animal cells, makes site-specific changes to intact genomes. CRISPR-Cas9 is more precise, more efficient, and less expensive than other genome editing tools and, as a result, has facilitated a wide range of studies that were previously unachievable.