The Winklevoss twins are moving ahead with plans to list a Bitcoin exchange fund on Nasdaq.

The brothers, Tyler and Cameron, filed an amendment with the Securities and Exchange Commission on Thursday seeking to open the Winklevoss Bitcoin Trust as an exchange traded fund (ETF) that would track the price of Bitcoin. The IPO for the stock would put 1 million shares up for sale. Each share is worth one-fifth of a bitcoin, or about $90, based on Friday's price. Tyler and Cameron Winklevoss own about 1% of the world's bitcoins.

The twins, who are best known for a disputed role in Facebook's founding, first proposed the idea last July. It was viewed as a long shot because Nasdaq had never tracked a virtual currency before. Most ETFs are linked to stocks, commodities and bonds. ETFs are similar to a mutual fund, but they are sold and traded as individual stocks.

The SEC approval, which came Thursday, occurred a day after the agency warned that investments related to Bitcoin might have a "heightened risk of fraud." During the time since the twins proposed their ETF, Mt. Gox — the best-known trading exchange for Bitcoin — imploded, leading U.S. regulators to mull legislation related to the currency.