Marlon Jackson just broke down in tears during an interview ... and walked off the set -- after explaining how his mother has been forcibly cut off from the rest of her family.
Marlon -- along with brothers Tito and Jackie -- were sitting for an interview with "The Insider" when Marlon choked up.
Marlon explained, "We've been told that our mother is safe. She's doing well. But we have not spoken to her. We have not talked to her and I don't know where my mother is."
At that point, Marlon grew incensed, saying, "All i know is she is in Arizona, and whatever doctor is saying I can't talk to my mother you call ME!" Marlon then stormed off.
Randy Jackson -- one of the siblings who helped whisk away Katherine to Arizona -- has said Katherine was under doctor's orders to escape the constant frenzy of the Jackson home, and relax.
Clearly, Marlon didn't get the memo.