Doctors have reported on the first attempts in the United States to use gene editing to help patients fight cancer.

The doctors say one form of gene editing appeared to be safe when tested in three patients. But it is not yet known what long-term effects the method will have on cancer treatment or patient survival rates.

A gene editing tool called CRISPR/Cas9 was used in the tests, which were recently reported in a medical study. The method was discovered in recent years as a way to change the genetic material that make up a person’s DNA.