The "Euphoria" and "Grey's Anatomy" star, who revealed his diagnosis last Tuesday, sat down with "Good Morning America" to talk about his incurable condition.

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.