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.

ALS has robbed Eric Dane of functioning in his right arm and is creeping through his left, the veteran actor said in an emotional interview about the diagnosis he revealed in April.

In an exclusive interview with Diane Sawyer for ‘Good Morning America,’ the beloved ‘Euphoria’ and ‘Grey’s Anatomy’ star breaks his silence on the heartbreaking diagnosis.