One thing cells must do in order to become cancerous is to overthrow the normal checks on their growth. As a part of this process, the stringent controls on things like copying and repairing DNA start to break down. As a result, tumors often contain chromosomal rearrangements, which are places where genes are cut and pasted back together in ways that they shouldn’t be.

In some cases, the breaks bring two genes together in a way that causes what are called “driver mutations,” forming a fusion protein that pushes the cells further along the road to malignancy. For some types of cancer, nearly every tumor contains one of these chromosome breaks, making its fusion genes a hallmark of that cancer. A group of researchers at the University of Pittsburgh School of Medicine just took advantage of this specificity by targeting the fusion genes to attack cancer cells and take them down.

The work relies on the CRISPR-Cas system, which is used by bacteria to recognize foreign DNA (like that of a virus) and chop it up. We’ve since learned how to target any DNA sequence, making it a formidable means of gene editing. Here, the researchers used a CRISPR-Cas system to make a nick—a single stranded cut in the double helix—in the tumor DNA, right at the point where two genes are fused.