(Adds details, comments from traders, recasts lead)
NEW YORK Jan 30 (Reuters) - Sarepta Therapeutics Inc became the second company in as many days to see its shares plunge as a result of a Twitter hoax on Wednesday after a user posed as an influential short-seller and alleged improprieties at the biopharmaceutical company.
Sarepta Therapeutics shares plummeted 9.9 percent in a matter of seconds after someone with the Twitter user name @citreonresearc alleged improprieties at the company.
The drop mirrored a similar incident on Tuesday, when Audience Inc fell more than 25 percent following tweets that at a glance looked to be from Muddy Waters, another short-selling firm.
Just as Muddy Waters confirmed it did not send such a tweet about Audience, so too did Andrew Left, the California-based investor who runs Citron, said his company did not send a message about Sarepta.
The twin incidents targeted a pair of Nasdaq stocks that are not among the most actively traded on a daily basis, showing how certain company shares are vulnerable to information posted on social media networks, even if the information is misleading. Sarepta, on average, trades about 1.4 million shares a day; by contrast, average daily volume in chipmaker Intel Corp is about 52 million shares.
"You need a more volatile stock for this kind of manipulation - obviously if you were to try it on IBM it wouldn't work," said Joe Saluzzi, co-manager of trading at Themis Trading in Chatham, New Jersey. "It speaks to the structure of the market where shallow pools of liquidity can be easily pierced on bad news, even if the news is fake."
The U.S. Securities and Exchange Commission had no comment.
Twitter did not respond to an emailed request for comment on the messaging activity, but the @citreonresearc account has been suspended. The Twitter account @Mudd1waters, whose Twitter home page looks like Muddy Waters, was still active.
The tweet, with the user name "@citreonresearc," alleged that drug trial results from the biopharmaceutical company had been tainted and doctored, according to screen shots of the posting captured by Twitter users. Sarepta was not immediately available for comments on the trading activity or the account, which has since been suspended.
Matt, a trader in San Diego who did not want to give his last name, but who goes by the handle @given2tweet on Twitter, said, "There's a real severity to that tweet. It's at the core of what the company is, is this one trial."
Sarepta shares recovered after the brief fall. It was trading up 1.4 percent to $27.56 on volume of about 3.5 million shares in afternoon trading. More than 700,000 shares traded in the one minute when the stock suffered its steep decline.
The company, which has a market cap of about $692 million, is volatile, moving more than 1 percent in six of the past seven sessions. It jumped 477 percent in 2012 alone.
Audience went public in May 2012 and has a market cap of $254 million as of Tuesday's closing price, rising about 15 percent so far this year. The stock's 50-day average daily volume is slightly more than 160,000 shares, but that rose to more than 800,000 on Tuesday.
"Not really an efficient market when fake twitter accounts can crash stocks," Tim McClure, equity trader at SMB Capital in New York, wrote on his Twitter page.
Shares of Audience were briefly halted on Tuesday, but Sarepta shares did not reach a threshold necessary to trigger a halt. A Nasdaq spokesperson was not available for comment. (Additional reporting by Daniel Bases and Emily Flitter; Editing by David Gaffen and Leslie Gevirtz)