(Bloomberg) -- The biggest money managers are asking U.S. Congress to fix a largely frozen part of the mortgage-bond market with a step that would echo moves taken during the Great Depression.

The Association of Institutional Investors, whose members include Pacific Investment Management Co., Fidelity Investments and Schroders Plc, asked lawmakers in a letter today to create an “unambiguous fiduciary standard” for trustees of mortgage bonds without government backing as a Senate committee considers a bill that would overhaul the $9.4 trillion home-loan market by replacing U.S.-controlled Fannie Mae and Freddie Mac.

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