This week, the ratings of four biotechnology stocks on Portfolio Grader are down. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

EPIRUS Biopharmaceuticals, Inc.’s (EPRS) rating falls to a D (“sell”) this week, down from C (“hold”) the week prior. For more information, get Portfolio Grader’s complete analysis of EPRS stock.

This week, Trius Therapeutics, Inc. (TSRX) drops from a C to a D rating. Trius Therapeutics is a biopharmaceutical company. The stock receives F’s in Earnings Growth, Earnings Momentum and Equity. Cash Flow and Sales Growth also get F’s. To get an in-depth look at TSRX, get Portfolio Grader’s complete analysis of TSRX stock.

This is a rough week for Intrexon Corporation (XON). The company’s rating falls to D from the previous week’s C. The stock gets F’s in Earnings Momentum, Earnings Revisions and Equity. As of Sept. 10, 2014, 10.3% of outstanding Intrexon Corporation shares were held short. For more information, get Portfolio Grader’s complete analysis of XON stock.

Portola Pharmaceuticals, Inc. (PTLA) gets weaker ratings this week as last week’s C drops to a D. The stock gets F’s in Equity, Cash Flow and Sales Growth. To get an in-depth look at PTLA, get Portfolio Grader’s complete analysis of PTLA 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.