WASHINGTON (AP) — Altering human heredity? In a first, researchers safely repaired a disease-causing gene in human embryos, targeting a heart defect best known for killing young athletes — a big step toward one day preventing a list of inherited diseases.

In a surprising discovery, a research team led by Oregon Health and & Science University reported that embryos can help fix themselves if scientists jump-start the process early enough.

It’s laboratory research only, nowhere near ready to be tried in a pregnancy. But it suggests that scientists might alter DNA in a way that protects not just one baby from a disease that runs in the family, but his or her offspring as well. And that raises ethical questions.

“I for one believe, and this paper supports the view, that ultimately gene editing of human embryos can be made safe. Then the question truly becomes, if we can do it, should we do it?” said Dr. George Daley, a stem cell scientist and dean of Harvard Medical School. He wasn’t involved in the new research and praised it as “quite remarkable.”

“This is definitely a leap forward,” agreed developmental geneticist Robin Lovell-Badge of Britain’s Francis Crick Institute.

Today, couples seeking to avoid passing on a bad gene sometimes have embryos created in fertility clinics so they can discard those that inherit the disease and attempt pregnancy only with healthy ones, if there are any.

Gene editing in theory could rescue diseased embryos. But so-called “germline” changes — altering sperm, eggs or embryos — are controversial because they would be permanent, passed down to future generations. Critics worry about attempts at “designer babies” instead of just preventing disease, and a few previous attempts at learning to edit embryos, in China, didn’t work well and, more importantly, raised safety concerns.

In a series of laboratory experiments reported in the journal Nature, the Oregon researchers tried a different approach.

They targeted a gene mutation that causes a heart-weakening disease, hypertrophic cardiomyopathy, that affects about 1 in 500 people. Inheriting just one copy of the bad gene can cause it.