Why Apple stock just got a rare downgrade to 'sell'

Apple CEO Tim Cook.

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• Apple stock received a rare downgrade to 'Sell' from an analyst on Tuesday.
• The downgrade highlights premium valuation concerns and some negative headlines in recent months.
• Of the more than 60 analysts that cover Apple on Wall Street, only four have "sell" ratings.

Apple received a rare downgrade to "Sell" from Wall Street on Tuesday.

MoffettNathanson downgraded the world's largest company from "Neutral" and set a $188 price target...