NEW YORK (MarketWatch) -- The Winklevoss Bitcoin Trust on Wednesday filed to sell 20.1 million shares on the Nasdaq exchange. The shares represent units of interest in the bitcoin exchange-traded fund launched by Tyler and Cameron Winklevoss, the twins who are best known for their legal dispute with Mark Zuckerberg over Facebook's FB, -1.73% origins. The ETF will be listed under the symbol COIN, according to the filing. The Winklevoss brothers also launched a bitcoin index - the Winkdex - in February. Bitcoin is a decentralized virtual currency that has attracted attention for its big price swings.