In 2006, Lesnar pursued a career in mixed martial arts (MMA), as he signed with Hero's and won his first fight, against Min-Soo Kim, in June 2007. He then signed with the Ultimate Fighting Championship (UFC) the following October. Lesnar lost in his UFC debut against Frank Mir and then won his second fight against Heath Herring. In November 2008, Lesnar defeated Randy Couture to become the UFC Heavyweight Champion. Shortly after a successful title defense in a rematch with Mir, Lesnar was sidelined due to diverticulitis. He would return at "UFC 116" to defeat Interim UFC Heavyweight Champion Shane Carwin and unify the heavyweight championships, becoming the undisputed UFC Heavyweight Champion in the process. Lesnar then lost the championship to Cain Velasquez at "UFC 121". In 2011, he was once again sidelined due to diverticulitis and underwent surgery in May 28. Lesnar returned at "UFC 141" in December, losing to Alistair Overeem and promptly retiring from MMA. Lesnar was a box office sensation in UFC, as he took part in some of the bestselling pay-per-view events in UFC history, including "UFC 100" and "UFC 116".