The first attempt in the United States to use a gene editing tool called CRISPR against cancer seems safe in the three patients who have had it so far, but it’s too soon to know if it will improve survival, doctors reported Wednesday.

The doctors were able to take immune system cells from the patients’ blood and alter them genetically to help them recognize and fight cancer, with minimal and manageable side effects. The treatment deletes three genes that might have been hindering these cells’ ability to attack the disease, and adds a new, fourth feature to help them do the job.

“It’s the most complicated genetic, cellular engineering that’s been attempted so far,” said the study leader, Dr. Edward Stadtmauer of the University of Pennsylvania in Philadelphia. “This is proof that we can safely do gene editing of these cells.”