LOS ANGELES (AP) — Researchers are reporting a key advance in using stem cells to repair hearts damaged by heart attacks.

In a study in Miami and Baltimore, stem cells donated by strangers proved as safe and effective as patients’ own cells for helping restore heart tissue, allowing the heart to pump more effectively.

The work involved just 30 patients, but proves the concept that anyone’s cells can be used to treat such cases. Doctors are excited because this suggests that stem cells could be banked for off-the-shelf use in patients after heart attacks the way blood is saved now.

Stem cell therapy reduced the scarred area of patients’ hearts by about one third. Results were discussed Monday at a heart conference in California.