Dane -- who shot to fame amid six seasons on the hit ABC drama "Grey's Anatomy," where he played Dr. Mark Sloan, affectionately known as "McSteamy" -- said his symptoms began over one year ago, when he began to experience weakness in his right hand.

Eric Dane, an actor best known for playing Dr. Mark Sloan, aka "McSteamy," on "Grey's Anatomy," discussed his health challenges after being diagnosed with ALS.

Actor Eric Dane has opened up about his battle with amyotrophic lateral sclerosis (ALS) and is revealing the one symptom — the very first one — he experienced, but didn’t think much of at the time it happened. The former “Grey’s Anatomy” star, 52, revealed this important fact to Diane Sawyer in a new interview on “Good Morning America” Monday.