When we blew up Skynet Central, and rescued Kyle I was injured. Took a piece of rebar through the heart courtesy of a brand new T-800. Marcus chose to save me. He donated his heart to replace mine.

He's also a Terminator, but not like we've seen before. He's human, at least his brain, and heart are. They wrapped that up in a metal skeleton, and sent it off to find me. It almost worked too. Guess Skynet still has trouble accounting for the human tendency to react badly to being played for a fool.