Shares in technology giant Apple have dropped about 4% in early New York trading, after boss Steve Jobs took indefinite medical leave.While Mr Jobs is continuing as chief executive and will be involved in any major decisions, day-to-day running has passed to Tim Cook.
The news came on Monday, a US public holiday, when markets were closed.
Apple shares are also traded in Frankfurt, where they fell 9% on Monday immediately after the announcement.
The share price initially dropped 6% at the opening bell in New York, before immediately recovering.
The movements come ahead of Apple's first-quarter results, due after the close of trading in New York on Tuesday.
Medical history Despite Mr Jobs' previous ill health, the company's stock market value has approximately quadrupled in the past two years.
In late 2008 to mid-2009 Mr Jobs was absent from Apple for six months to have a liver transplant.
It was part of a series of treatments he has undergone for pancreatic cancer. He was first diagnosed in 2004 and underwent surgery later that year to remove a tumour from his pancreas."US investors are concerned about his absence," says Yair Reiner, stock analyst at New York investment firm Oppenheimer & Co.
"But the ups and downs of his health over the last couple of years have allowed investors to partly discount his departure into the price of their shares."
He notes that the typical pattern following previous health scares has been for a sudden sharp stock sell-off to be followed by a return to trading based on the company's performance.
In an e-mail to staff, Mr Jobs said he would be back at work as soon as he could.
The letter "leaves everything to the imagination" said Mr Reiner, adding that the company had provided no guidance as to whether it would be a short break or the prelude to a permanent departure.
Mr Cook is currently the firm's chief operating officer. He has run the company in the past during Mr Jobs' absence.
According to Mr Reiner, markets have a lot of confidence in the management abilities of Mr Jobs' stand-in, although he has not yet had the opportunity to demonstrate whether can provide the same visionary leadership as his boss.
Mr Jobs is a central figure at Apple and is widely credited as the architect of its current run of success based around products such as the iPad and iPhone.
The absence comes as Apple is rumoured to be preparing to launch the second version of its iPad - the successor to the tablet computer it launched in 2010.
With the product line-up for the next two to three years already set, Mr Reiner said that the real impact of a permanent departure of the Apple head would only be felt some years down the line.