(Bloomberg) -- You know a market's tough to crack when the world's largest asset manager says it's struggling to gain traction.

BlackRock's efforts to sell ETFs focused on currencies have failed due to a lack of institutional interest, according to Mark Wiedman, global head of the company's iShares ETF business and of index investments. That's because while retail investors get hit by fees or embedded spreads when they trade currencies, professionals can move millions for less than a basis-point cost. Yet ETFs need institutional use to thrive.

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