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.

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.