Following a prominent fund's "breaking of the buck"--that is, a decline in its net asset value below par--in September, investors began to withdraw funds in large amounts from money market mutual funds that invest in private instruments such as commercial paper and certificates of deposit.

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The willingness to take a chance on blemished credits pushed below-investment grade securities above par value for the first time since before the financial crisis, up from a nadir of 55 cents on the dollar in December of 2008, according to Bank of America Merrill Lynch.

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