NEW YORK ( TheStreet) -- Shares of Achillion Pharmaceuticals (Nasdaq: ACHN) were gapping down Thursday morning with an open price 19.7% lower than Wednesday's closing price. The stock closed at $9.50 Wednesday and opened today's trading at $7.63. The average volume for Achillion has been 1.5 million shares per day over the past 30 days. Achillion has a market cap of $745.9 million and is part of the health care sector and drugs industry. Shares are up 18.8% year to date as of the close of trading on Wednesday. Achillion Pharmaceuticals, Inc., a biopharmaceutical company, discovers, develops, and commercializes anti-infective drug therapies in the United States and internationally. TheStreet Ratings rates Achillion as a sell. The company's weaknesses can be seen in multiple areas, such as its deteriorating net income and feeble growth in its earnings per share. You can view the full Achillion Ratings Report. Get more investment ideas from our investment research center.