This week, the overall grades of nine biotechnology stocks are lower, according to the Portfolio Grader database. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

Merrimack Pharmaceuticals, Inc. (MACK) is on the decline this week, earning a D (“sell”) after receiving a C (“hold”) last week. Merrimack is a biotechnology company. In Portfolio Grader’s specific subcategories of Cash Flow and Sales Growth, MACK also gets F’s. As of Dec. 18, 2013, 13.2% of outstanding Merrimack Pharmaceuticals, Inc. shares were held short. For a full analysis of MACK stock, visit Portfolio Grader.

Slipping from a C to a D rating, Genomic Health, Inc. (GHDX) takes a hit this week. Genomic Health is a life science company, which is focused on the development and commercialization of genomic-based clinical diagnostic tests for cancer that allow physicians and patients to make individualized treatment decisions. The stock also gets an F in Earnings Revisions. The stock price has fallen 15.8% over the past month, worse than the 1.3% decrease the Nasdaq has seen over the same period of time. For more information, get Portfolio Grader’s complete analysis of GHDX stock.

Acorda Therapeutics, Inc. (ACOR) earns a D this week, falling from last week’s grade of C. Acorda Therapeutics is a commercial stage biopharmaceutical company dedicated to the identification, development and commercialization of novel therapies that improve neurological function in people with multiple sclerosis (MS), spinal cord injury and other disorders of the central nervous system. The stock gets F’s in Earnings Revisions and Sales Growth. Share prices fell 9.8% over the past month. To get an in-depth look at ACOR, get Portfolio Grader’s complete analysis of ACOR stock.

Amarin Corporation Plc Sponsored ADR (AMRN) gets weaker ratings this week as last week’s C drops to a D. Amarin focuses on developing the treatment for cardiovascular disease in the field of lipid science. The stock receives F’s in Earnings Growth, Earnings Revisions, Equity and Cash Flow. Wall Street appears to agree with the stock downgrade, with share prices dropping 13.9% over the past month. As of Dec. 18, 2013, 17.3% of outstanding Amarin Corporation Plc Sponsored ADR shares were held short. For more information, get Portfolio Grader’s complete analysis of AMRN stock.

The rating of Northwest Biotherapeutics, Inc. (NWBO) declines this week from a C to a D. The stock also rates an F in Cash Flow. Northwest Biotherapeutics engages in discovering, developing, and commercializing immunotherapy products to treat cancers in the United States. For a full analysis of NWBO stock, visit Portfolio Grader.

Exelixis, Inc. (EXEL) ratings are on the decline this week as the company earns an F (“strong sell”). Last week, it received a D (“sell”). Exelixisis a development-stage biotechnology company dedicated to the discovery and development of small-molecule therapeutics for the treatment of cancer and other serious diseases. The stock receives F’s in Earnings Growth, Earnings Momentum and Equity. Margin Growth and Sales Growth also get F’s. As of Dec. 18, 2013, 25.3% of outstanding Exelixis, Inc. shares were held short. To get an in-depth look at EXEL, get Portfolio Grader’s complete analysis of EXEL stock.

Trius Therapeutics, Inc.’s (TSRX) rating weakens this week, dropping to a D versus last week’s C. Trius Therapeutics is a biopharmaceutical company. The stock gets F’s in Earnings Growth, Earnings Momentum and Equity. Cash Flow and Sales Growth also get F’s. For more information, get Portfolio Grader’s complete analysis of TSRX stock.

This is a rough week for Discovery Laboratories, Inc. (DSCO). The company’s rating falls to D from the previous week’s C. Discovery Laboratories is a biotechnology company focused on developing products for the treatment of respiratory disease. The stock gets F’s in Equity and Cash Flow. To get an in-depth look at DSCO, get Portfolio Grader’s complete analysis of DSCO stock.

This week, Achillion Pharmaceuticals, Inc. (ACHN) drops from a C to a D rating. Achillion focuses on the discovery, development and commercialization of innovative treatments for infectious diseases. The stock also gets an F in Equity. As of Dec. 18, 2013, 18% of outstanding Achillion Pharmaceuticals, Inc. shares were held short. For a full analysis of ACHN stock, visit Portfolio Grader.

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.