The overall ratings of six Biotechnology stocks are down on Portfolio Grader this week. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

This week, Achillion (NASDAQ:ACHN) falls to a D (“sell”), worse than last week’s grade of C (“hold”). Achillion focuses on the discovery, development and commercialization of innovative treatments for infectious diseases. In Portfolio Grader’s specific subcategories of Equity and Sales Growth, ACHN also gets F’s. As of Jan. 30, 2013, 16.3% of outstanding Achillion shares were held short. For a full analysis of ACHN stock, visit Portfolio Grader.

Raptor Pharmaceutical (NASDAQ:RPTP) earns an F (“strong sell”) this week, moving down from last week’s grade of D (“sell”). Raptor Pharmaceutical researches existing drugs to improve them through specialized drug targeting methods and formulation changes. The stock gets F’s in Earnings Revisions, Equity, and Cash Flow. The stock price has fallen 7.4% over the past month, worse than the 6.5% increase the Nasdaq has seen over the same period of time. As of Jan. 30, 2013, 13.8% of outstanding Raptor Pharmaceutical shares were held short. For more information, get Portfolio Grader’s complete analysis of RPTP stock.

The rating of Cytori Therapeutics (NASDAQ:CYTX) slips from C to a D. Cytori Therapeutics develops, manufactures and sells medical products to enable the practice of regenerative medicine. The stock gets F’s in Cash Flow and Sales Growth. As of Jan. 30, 2013, 12.2% of outstanding Cytori Therapeutics shares were held short. To get an in-depth look at CYTX, get Portfolio Grader’s complete analysis of CYTX stock.

Cepheid (NASDAQ:CPHD) is having a tough week. The company’s rating falls from C to a D. Cepheid is a molecular diagnostics company that engages in developing, manufacturing, and marketing integrated systems for testing in the clinical market, as well as for application in legacy biothreat, industrial, and partner markets. The stock gets F’s in Earnings Growth, Earnings Revisions, and Margin Growth. As of Jan. 30, 2013, 12% of outstanding Cepheid shares were held short. For a full analysis of CPHD stock, visit Portfolio Grader.

This week, Rigel Pharmaceuticals’ (NASDAQ:RIGL) rating worsens to an F from the company’s D rating a week ago. Rigel Pharma discovers and develops novel, small molecule drugs in four lead product development programs: allergy/asthma, autoimmune disorders, hepatitis C, and cancer. The stock gets F’s in Equity, Cash Flow, and Sales Growth. For more information, get Portfolio Grader’s complete analysis of RIGL stock.

The rating of Ironwood Pharmaceuticals (NASDAQ:IRWD) declines this week from C to a D. Ironwood Pharmaceuticals discovers, develops, manufactures, and commercializes marketed drugs. The stock also gets an F in Equity. As of Jan. 30, 2013, 11.4% of outstanding Ironwood Pharmaceuticals shares were held short. To get an in-depth look at IRWD, get Portfolio Grader’s complete analysis of IRWD stock.

Louis Navellier’s proprietary Portfolio Grader stock ranking system assesses roughly 5,000 companies every week based on a number of fundamental and quantitative measures. Stocks are given a letter grade based on their results — with A being “strong buy,” and F being “strong sell.” Explore the tool here.