The gene-editing system CRISPR is the hottest subject in biology because of the technique’s ability to change DNA letters and potentially cure genetic disease. A startup called Homology Medicines says it has a way to do that. The Bedford, Massachusetts, company has raised an impressive $127 million to treat genetic diseases using viruses it claims are capable of efficiently repairing human genes, all on their own. If its claim is true, Homology may have hit on the safest, simplest way yet to change genes in the human body—one that doesn’t require slashing open a person’s DNA strands, as CRISPR does.