Sometime this year, people in the US and Europe will start getting treated for diseases using the gene-editing tool CRISPR, but a big question remains—will it actually work?

Our primate cousins may hold the answer.

The first use of CRISPR to edit human cells in a dish was reported in 2013. It’s since been touted as an easy way to alter people’s DNA, promising to banish what are currently lethal or lifelong maladies with a single treatment that fixes them at the genetic root.

But since cutting DNA would permanently change someone’s genome, scientists need to make sure CRISPR is safe and effective before using it in people. Mice aren’t always an accurate way to predict how humans will react to a new therapy, so scientists often turn to monkeys as the gold standard in research that’s headed for the clinic.

At the moment, there isn’t much published data on CRISPR’s use in monkeys, but early results from small studies point to the prospect of a cure for some diseases—and to challenges in other cases.

Some of the ongoing CRISPR experiments in monkeys involve blood disorders like sickle-cell disease and beta thalassemia. CRISPR could be ideal for both diseases, scientists think, because each is caused by mutations in a single gene that makes hemoglobin, the protein in red blood cells that transports oxygen throughout the body. Both diseases could be repaired with a single genetic snip.

The idea is to use CRISPR to knock out a certain section of DNA and flip on a switch to produce a fetal type of hemoglobin that’s usually shut off after birth. This would give patients enough healthy hemoglobin to wipe out any symptoms of disease.

Researchers like Cynthia Dunbar at the National Institutes of Health are testing this approach in the blood-forming stem cells found in bone marrow. These cells are extracted from monkeys, altered with CRISPR in a lab, and then infused back into the monkeys’ marrow to grow and make new, healthy blood cells.

Raising the level

For a treatment to be effective, a certain percentage of cells need to get edited. In the monkeys she’s studying, Dunbar says, things look good initially, but after three to four months, only about 5 percent of cells have the necessary edit. To alleviate sickle-cell, “you’d probably need something approaching 20 percent,” she says.