The Securities and Exchange Commission has rejected a proposal to package bitcoin into an exchange traded fund, the latest indication that regulators are still uneasy with the volatile and largely unpoliced cryptocurrency market.

The commission’s vote affirms an earlier decision in 2017 by the SEC’s staff to deny the proposal, for which Cameron and Tyler Winklevoss first sought approval several years ago. Cryptocurrency traders and exchanges have hoped that an exchange-traded product would make the virtual currency more attractive to Wall Street and retail investors.