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Shares in online retail giant Amazon have risen above the $1,000 mark for the first time.

The shares touched $1,001.2 at one point on Tuesday before slipping back to $996.7. It originally listed its shares in May 1997 for just $18 each.

Amazon now has a market capitalisation of about $478bn, which is more than twice that of Wal-Mart.

After starting as a bookseller, it has steadily expanded to become much broader-based retailer.

According to consultancy Slice Intelligence, Amazon now accounts for about 43% of all online sales in the US.

Amazon is now the fourth-largest US company by market capitalisation, behind Apple, Google owner Alphabet, and Microsoft.

Alphabet's class A shares were also close to hitting four figures on Tuesday, trading at $996, meaning the company is worth around $681bn.

Despite the rise in Amazon's share price, the Nasdaq index - of which Amazon is a member - closed down 7 points or 0.11% at 6,203.19.

The Dow Jones Industrial Average dropped 50.81 points to 21,029.47 while the wider S&P 500 index slid 2.91 points to 2,412.91 as investors picked their way through a raft of US economic data.

Figures from the Commerce Department showed that consumer spending posted its biggest rise for four months in April, climbing 0.4%. That followed an upwardly revised 0.3% increase in March.

Separate data showed the personal consumption expenditures (PCE) price index rose by 0.2% in April, following a 0.2% fall in March. Prices in April were up 1.7% from a year earlier, although that was down from March's figure of 1.9%.

The so-called core PCE price index - which strips out food and energy prices, and is the Federal Reserve's favoured inflation measure - showed a 1.5% annual rise, compared with a 1.6% increase in March.

The Fed targets a rate of 2% for the core PCE index. Many analysts expect that the US central bank will raise interest rates when it meets next month.