ATLANTA, Georgia (CNN) -- For the first time in the United States, stem cells have been directly injected into the spinal cord of a patient, researchers announced Thursday.

Doctors injected stem cells from 8-week-old fetal tissue into the spine of a man in his early 60s who has advanced ALS, or amyotrophic lateral sclerosis. It was part of a clinical trial designed to determine whether it is safe to inject stem cells into the spinal cord and whether the cells themselves are safe.

ALS is a fatal neurodegenerative disease that causes the deterioration of specific nerve cells in the brain and spinal cord called motor neurons, which control muscle movement. About 30,000 Americans have ALS at any given time, according to the ALS Association.

There is no cure for ALS, which is better known as Lou Gehrig's disease, named after the New York Yankees' first baseman and Hall of Famer who retired from baseball in the 1930s after being diagnosed with the disease.