In 1911, as a small child, Mark Rein left Russia with his parents, who were escaping from the tsarist police.
He was more sympathetic to the Soviet Union than his father, and for a while supported the idea of a unification of social-democratic and communist parties.
In 1936, he went to Spain to support the Republican antifascist forces during the Spanish Civil War and became a friend to Willy Brandt there, whom he met in Barcelona.
He was spirited to Russia, apparently with the intention of using him in the show trial of Alexey Rykov and Nikolai Bukharin.
In spite of frantic efforts by Rafail Abramovich and western socialist supporters, Rein was never seen alive again and is thought to have been murdered by the OGPU.