Stock index futures extended their fall on Monday, pulled lower by Apple Inc
Apple's shares traded in Frankfurt
The U.S. markets are closed for a holiday on Monday and will resume trading on Tuesday.
During Jobs' leave of absence, Chief Operating Officer Tim Cook will be responsible for day to day operations. Jobs will continue as CEO and will be involved in major strategic decisions for the company.
He did not say for how long he would be on leave.
Honestly, the effect on the company will probably not be that great in terms of fundamentals, said Richard Windsor, global technology specialist at Nomura.
Perception of the company is another matter. Steve Jobs is seen by the market to be a major force in Apple's strategic direction. If his pancreatic cancer has returned, one could be quite worried.
Jobs underwent a liver transplant while on leave, returning to the company in late June.
(Reporting by Dominic Lau and Georgina Prodhan; Editing by David Holmes)