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 Wednesday 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...