Question: Who was a creator of a science-fiction role-playing game along with Phil McGregor and an American game designer, notable for his work on "Space Marines" ?

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Rob Heinsoo: Rob Heinsoo (born 1964 ) is an American tabletop game designer.  He has been designing and contributing to professional role-playing games, card games, and board games since 1994.  Heinsoo was the lead designer on the 4th Edition of "Dungeons & Dragons" (2008), and is co-designer of the 13th Age roleplaying game along with Jonathan Tweet.  He has also designed and contributed to role playing, miniatures and card games, and a computer game.

SSDC, Inc.: SSDC, Inc. is a small publisher of role-playing games and poetry collections, based in Rochester, New York.  Their two product lines are the Battlelords of the 23rd Century science-fiction role-playing game, and the Blood Dawn post-apocalyptic RPG.  SSDC was formed by Michael Osadciw and Aaron Thies, two former employees of Optimus Design Systems (ODS).  SSDC acquired the licenses to these products from Optimus Design Systems in September 1999.

Space Opera (role-playing game): Space Opera is a science-fiction role-playing game created by Edward E. Simbalist, A. Mark Ratner, and Phil McGregor in 1980 for Fantasy Games Unlimited.  While the system is applicable to the whole genre of science fiction, "Space Opera" had a default setting intended to be used as generic science fiction role-playing game rules, the focus being on creating space opera themed adventures.

Killer: The Game of Assassination: Killer: The Game of Assassination is a live action role-playing game derived from the traditional "assassin" game.  Created by American game designer Steve Jackson and first published in 1981 or 1982 by his own game company, Steve Jackson Games, "Killer" is one of the oldest set of rules having been officially published to play the assassin game.

Tales from the Floating Vagabond: Tales from the Floating Vagabond is a science-fiction role-playing game by Lee Garvin, published by Avalon Hill in 1991.  It has the tagline "Ludicrous Adventure in a Universe Whose Natural Laws Are Out To Lunch".

Warhammer 40,000 Collectible Card Game: Warhammer 40,000 Collectible Card Game, sometimes denoted WH40KCCG is a collectible card game released in 2001 by Sabertooth Games.  It is set in the fictional "Warhammer 40,000" universe.  The players may select various factions around which they could base their personalized deck.  These factions include Orks, Space Marines, Chaos, Eldar, Tyranids, Imperial Guard, and Dark Eldar; there were also sub-factions (such as individual Space Marines chapters).

Optimus Design Systems: Optimus Design Systems (ODS) was a small publisher of role-playing games (RPGs), based in Buffalo, New York.  The company produced two products, the "Battlelords of the 23rd Century" science-fiction role-playing game, and the "Blood Dawn" post-apocalyptic RPG.  ODS produced the first of several editions of "Battlelords of the 23rd Century" in 1990, followed by several supplements.  In addition, it opened two game stores, one in Buffalo, New York and one in Williamsville, New York.  ODS was primarily run by its founder, Lawrence R. Sims, who was also the primary author of its products.  The company eventually succumbed to financial difficulties.  SSDC, Inc. acquired the licenses to these products in 1999.

Matt James (game designer): Matt James (born 1981) is an American game designer and a decorated veteran of the United States Army.  As a game designer, James is best known for his online and print works for the "Dungeons & Dragons" fantasy role-playing game published by Wizards of the Coast, "Pathfinder" role-playing game by Paizo, and Privateer Press.  He has also designed game content for Wolfgang Baur's Kobold Press (formerly Open Design LLC).  In 2012 James won an ENnie Award for "" and has been nominated for several Origins Awards.  In 2014, James won an ENnie Award for "".

Forgotten Realms: Forgotten Realms is a campaign setting for the "Dungeons & Dragons" ("D&D") fantasy role-playing game.  Commonly referred to by players and game designers alike as "The Realms", it was created by game designer Ed Greenwood around 1967 as a setting for his childhood stories.  Several years later, Greenwood brought the setting to the "D&D" game as a series of magazine articles, and the first Realms game products were released in 1987.  Role-playing game products have been produced for the setting ever since, as have various licensed products including sword and sorcery novels, role-playing video game adaptations (including the first massively multiplayer online role-playing game to use graphics), and comic books.  The Forgotten Realms is one of the most popular "D&D" settings, largely due to the success of novels by authors such as R. A. Salvatore and numerous role-playing video games, including "Pool of Radiance" (1988), "Eye of the Beholder" (1991), "Baldur's Gate" (1998), "Icewind Dale" (2000) and "Neverwinter Nights" (2002).

A. Mark Ratner: A. Mark Ratner (born November 1948 in New Jersey ) is an American game designer, notable for his work on "Space Marines" and Space Opera.

Supporting paragraphs: A. Mark Ratner Space Opera (role-playing game)
Answer: Edward E. Simbalist