The researchers paid special attention to the interaction between the DNA and Cas9, the cutting protein. “Our previous work suggested that Cas9 might bind to its intended target DNA site with more energy than it needs, enabling unwanted cleavage of imperfectly matched off-target sites," study author Vikram Pattanayak said in a press release. To tweak that interaction, the researchers altered the number of amino acids that Cas9 uses to bind to the DNA. After testing 15 different iterations, they found one version that produced no detectable off-target effects. They named it SpCas9-HF1. By adding more amino acids to the mix, the researchers found that they could extend the range of DNA that the protein targets, making it more useful to scientists.