The actor, whose real name was Dearon Thompson, was found unresponsive at home Thursday morning in Los Angeles. His brother, Marshawn, tells us the family believes he died of a heart attack, but there is not yet an official cause of death.

The actor underwent major heart surgery in 2009 -- a 6-hour procedure to replace his aorta and fix a leaky heart valve.

Deezer D appeared on almost 200 episodes of ER from 1994 through 2009. Not only was Thompson an actor, but he was also a motivational speaker and a performer in the Christian and underground hip hop communities.

Thompson also had a role in 'Romy and Michele' and was known for his roles in the films "CB4" and "Fear of a Black Hat."

He was 55.