What’s more, they can do it without introducing any new and potentially disease-causing errors to the surrounding DNA sequence . The glitches occur in genes encoding an important enzyme complex responsible for the blood cells’ normal antimicrobial activity. In this case, it was a single typo in the gene sequence on the X chromosome. That’s because the guide RNA was truly specific enough to find and replace just the DNA sequence containing the single-letter typo. In fact, a clinical trial under way at NIH and other sites around the country is now testing a more-traditional approach to CGD gene therapy .