The logic is simple: alter the gene lines in an embryo’s eight or 16 cell stage (to, say, eliminate the gene for Tay-Sachs disease) and that change will occur in each of the resulting person’s trillions of cells – not to mention in the cells of their descendants.When combined with researchers’ growing understanding of the genetic links to various diseases, CRISPR could conceivably help eliminate a host of maladies in people before they are born.But many of the same scientists who have hailed CRISPR’s promise, including Doudna, also have warned of its potential dangers.At a National Academy of Sciences conference in Washington, D.C., in December 2015, she and about 500 researchers, ethicists and others urged the scientific community to hold off editing embryos for now, arguing that we do not yet know enough to safely make changes that can be passed down to future generations.Those at the conference also raised another concern: the idea of using the new technologies to edit embryos for non-therapeutic purposes.