While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a look at particularly stock-shaking analyst upgrades and downgrades -- just in case their reasoning behind the call makes sense. What: Shares of EXACT Sciences Corporation ( EXAS -0.38% ) slumped more than 4% today after Wedbush downgraded the biotechnology company from outperform to neutral. So what: Along with the downgrade, analyst Zarak Khurshid planted a price target of $14, representing about 1% worth of downside to yesterday's close. So while momentum traders might be attracted to EXACT's price strength in recent weeks, Khurshid's call could reflect a sense on Wall Street that its valuation is becoming a bit stretched. Now what: According to Wedbush, EXACT's risk/reward trade-off isn't too appealing at this point.