The study did not turn Crispr loose inside their bodies. The technology is relatively new, and some researchers fear that it could somehow edit the wrong gene. Instead, doctors removed T-cells, the soldiers of the immune system, from the patients’ blood, and used Crispr to delete three genes that might interfere with the cells’ ability to fight cancer or cause side effects in the patients.

Next, the researchers used a virus to arm the T-cells with what Dr. Stadtmauer called a warhead, which would direct the cells to attack a target called NY-ESO-1, a protein found on cancer cells but rarely on normal tissue. Then, after processing the cells to remove any traces of Crispr, the research team infused the cells — 100 million of them — back into the patients.

“Once we infused these cells, there was anywhere from a 10,000-fold to 100,000-fold increase in the amount of cells growing in the patients, which is exactly what we hoped for,” Dr. Stadtmauer said.

There were no ill effects, he said, and the cells did not die off.

“It’s good to have these cells hanging around doing serial surveillance for tumor,” he said. “We’re happy that up to nine months later we’re still seeing the cells.”

But are the cells fighting cancer? Dr. Stadtmauer said his team was tackling that question now, by mixing altered cells from the patients with cancer cells in petri dishes in the lab, to see if the T-cells kill the malignant ones. He said he expected to have the answer in a month.

The next phases of the study, involving more patients, will give the scientists a better sense of whether the treatment is working. If it is not, Dr. Stadtmauer said, the researchers can try to identify different targets and deploy other warheads.

“It really just opens up a whole world of this type of manipulation of cells to be directed to whatever the imagination can think of,” he said.