In Killing Floor (as well as it's sequal), Tripwire Interactive kept Masterson's past a mystery in reference to his mod origin (which makes canocity of this questionable). Masterson was at first the boss for KFMod before it's 2.0 update, where he was now made as a playable character in update 2.5, along with a rehabilitated Stalker named Claire. Some of the playable characters are also products of Horzine's specimen experiments. Kevin Clamely has facilities hidden everywhere, and with a near endless supply of clones at his disposal he is a threat to all remaining humans. The initial outbreak occurred in London, but has since spread all over Europe and possibly the entire world. Hence Kevin Clamely's assumed title of 'The Patriarch' and his references to the specimen as his 'children.' The female specimen were initially created at a separate facility and are believed to be clones of Kevin Clamely's (still-living) daughter, Rachel. The male specimen are all clones of Kevin Clamely's deceased son, produced by the Clot Mother. Contrary to popular belief, they are not actually undead, but are commonly referred to as 'Zeds' or 'zombies' all the same.
Specimen are the enemies faced by the Mercenaries and Survivors that were created by Kevin Clamely using the Horzine Corporation.
