Theranos, the beleaguered Silicon Valley blood-testing company, is being accused of securities fraud by a major investor who has sued contending “myriad deceptions, falsehoods and fraudulent conduct.”

Partner Fund Management, a San Francisco hedge fund, filed the lawsuit on Monday in the Delaware Court of Chancery, according to a letter sent to its investors.

The suit is under seal, but the fund confirmed in a statement that it was taking legal action against Theranos.