Valve games are built on its Source engine, which owes much of its success to mods and sequels. Gabe Newell is currently married to Lisa Newell and has two sons. Forbes reports that he has an estimated net worth of 1.1 billion dollars. Since the success of Half-Life, Newell and the Valve team have worked on numerous other video game series, such as Counter-Strike, Left 4 Dead and Portal. After a 13-year stint at Microsoft, Newell and fellow employee Mike Harrington left the company and used their profits to found a video-game studio, Valve Software. After a meeting with John Carmack, co-founder of id Software, they decided to license the Quake engine to create their first video game, Half-Life. Valve later acquired TF Software PTY Ltd, the makers of the Team Fortress mod for Quake, in May 1998 with the intent to create a standalone Team Fortress game.