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.

DNA is short for deoxyribonucleic acid. It is the substance that carries genetic information in the cells of living things. The CRISPR tool makes it possible to change DNA to add needed genes or take some away if they lead to problems.

Cancer researchers from the University of Pennsylvania Health System took immune system cells from the blood of the three patients. They changed the structure of the cells' genes to help them recognize and fight cancer. They were then put back in the patients. The researchers said the editing process was completed with no serious side effects.

The treatment removed three genes that might have been restricting the ability of the immune system cells to attack the cancer. A new, fourth gene was added to help the others work effectively.

Two of the patients suffer from multiple myeloma, a blood cancer, and the third has sarcoma, cancer that forms in connective or soft tissue. All had failed with repeated traditional cancer treatments.

"It's the most complicated genetic, cellular engineering that's been attempted so far," the study's leader, Edward Stadtmauer, told The Associated Press (AP). "This is proof that we can safely do gene editing of these cells."

So far, the cells have survived and have been reproducing as they should be, Stadtmauer said.

After two to three months, one patient's cancer continued to worsen, while the condition of another patient was unchanged. The third patient was treated too recently to effectively measure her progress. The researchers plan to expand the experimental treatment to 15 more patients.

Stadtmauer said that since the gene editing treatment is so new, it is not clear how soon major anti-cancer effects will be seen. Patients must be followed further and more tests will be needed, he said.

"It's very early, but I'm incredibly encouraged by this," one independent expert, Aaron Gerds, told an AP reporter.

Gerds is a cancer specialist at the Cleveland Clinic in Cleveland, Ohio. He added that other cell therapies for some blood cancers have worked very well, even "taking diseases that are uncurable and curing them." He said gene editing could provide a way to improve on those treatments.

Chinese scientists are reported to have attempted the CRISPR method on cancer patients. The U.S. study is the first known research to be completed outside China. It took researchers over two years to get approval from the U.S. government to try it.

More details about the study are to be provided at the yearly conference of the American Society of Hematology in December.

I'm Bryan Lynn.

The Associated Press reported on this story. Bryan Lynn adapted the report for VOA Learning English. George Grow was the editor.