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Believe it or not, Apple shares can go down.

The stock is selling off this afternoon, one day after the tech juggernaut surpassed Microsoft as the largest U.S. company ever, measured by stock-market value.

Shares are down 1.8% at $653.32, reversing earlier gains. The stock hit a fresh all-time high of $674.88 in midday trading, but started selling off as word circulated that a research outfit called Oracle Investment Research had downgraded Apple to hold from buy. (We'll admit, we had never heard of this firm until today when news of the downgrade starting spreading across Twitter).

"The hype concerns us," Oracle's Laurence Balter wrote in a note to clients, as he compared the current frenzy to when Microsoft and Cisco Systems previously were market-cap kings.

He cut his price target to $650 from $670.

Balter expressed concern about Apple's potential latest endeavor into the television business. Apple is seeking to shake up the TV market with a device that can carry live television, although it remains to be seen whether cable operators will bite.

"We believe entering the low margin world of television set top boxes and TV is fraught with margin danger," Balter says.

An Apple downgrade doesn't happen all that often, especially as most of Wall Street remains bullish. Last week Jefferies Group boosted its Apple price target to $900. And at least two analysts have predicted this year that Apple would top $1,000 a share, which would represent a 50% gain from Monday's closing level.

One of those analysts is Brian White of Topeka Capital Markets, who earlier today published a note calling Apple a "trillion-dollar baby." Apple surpassing the market value of Microsoft's 1999 peak removes "a key sentiment barrier" that had been keeping a lid on the stock in recent months, he says. And now investors should think of Apple's market cap in "trillions" and not "billions."

He has an $1,111 price target on Apple, which would carry its market cap above $1 trillion.

Another bullish theme to keep in mind is valuation. Apple is much cheaper than previous companies that had high market caps. Companies including Cisco Systems, Exxon Mobil, General Electric, Intel and Microsoft at one time had market caps that approached or topped $500 billion. But they also had P/E ratios that topped 60.

Apple's forward-year P/E ratio is 14.8, according to FactSet

"None of these companies delivered the annual [earnings] growth that Apple has achieved over the past seven years," White says.

Apple also doesn't dominate its broad sector like Microsoft, Intel or Cisco did at their peaks. Microsoft held more than a 90% market share for PC operating systems at one time, Intel had 80% of the PC processor market and Cisco had more than 70% of the networking market, White notes. Apple, on the other hand, only holds a fraction of the mobile market, while dominating the tablet market.

So while there are many reasons to be bullish, the big question is how long can Apple keep up the dominance? The bigger it gets, the harder it is to keep growing at a speed that will keep pleasing Wall Street.

Balter of Oracle Investment cited a famous quote in his report: "Anything that can't go on forever...will end."

-- Kevin Kingsbury contributed to this post.

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