Former 49ers wide receiver Dwight Clark announced Sunday night he has ALS, often referred to as Lou Gehrig’s disease.

“I have ALS, also known as Lou Gehrig’s disease,” Clark, 60, wrote on his blog, via San Jose Mercury News. “Those words are still very hard for me to say.”

ALS is a progressive neurodegenerative disease that affects nerve cells in the brain and the spinal cord, and debilitates the brain’s function of controlling muscle movement, per ALSA.org.