Apple Inc. CEO Steve Jobs, who started the company at age 21, announced his resignation from the post yesterday after being on medical leave since Jan. 17, Bloomberg reported.

Jobs has battled cancer since 2003 and had a liver transplant in 2009. He is succeeded by Tim Cook, Apple’s chief operating officer.

Cook joined the company in 1998 and has been running day-to-day operations during Jobs’ absence. Under Jobs, Apple became the second-most valuable company in the world by introducing devices that revolutionized the computer, music and mobile phone industries.