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, short for amyotrophic lateral sclerosis, is a degenerative neurological disorder that eventually leads to paralysis, taking away a person's ability to move, speak, swallow or even breathe, according to the National Institutes of Health.

The San Francisco-born star of ‘Grey’s Anatomy’ and ‘Euphoria’ has been on Good Morning America this week to discuss the frightening realities of amyotrophic lateral