When it comes to cancer, tumor suppressor genes are usually thought of as the “good guys.” These genes make proteins that protect and repair DNA in cells. If they stop functioning or there’s not enough, cancer risk goes up. But there can be too much of a good thing: when cells overexpress the gene EXO1—meaning that they make more of the protein than they should—it can degrade the DNA it’s supposed to repair. This causes damage that can disrupt the genome, which is a hallmark of cancer, according to a team of researchers from Penn State College of Medicine.

Tumor cells with high levels of EXO1 protein exhibit characteristics similar to cells with a BRCA mutation, a genetic code change known for its link to hereditary breast and ovarian cancers. That means that these tumor cells behave like BRCA-mutant cells—including their response to the chemotherapies and other drugs—even when there is no BRCA mutation present, a finding the researchers said hasn’t previously been established.