In a bold first-of-its-kind experiment, scientists have edited a person’s genes directly inside living tissue in an ambitious bid to cure a man of a rare, crippling genetic disorder.

While CRISPR has broken ground in things like editing human embryos and injecting patients with genetically edited cells, this alternative technique pioneers a new real-time approach to infusing a person’s blood with a gene-editing virus.

“For the first time, a patient has received a therapy intended to precisely edit the DNA of cells directly inside the body,” says CEO of Sangamo Therapeutics, Sandy Macrae, whose company is testing the experimental procedure.