Are Money Markets Funds Secure?

CNBC reported that the AXA Money Market Fund took a big haircut overnight.

AXA closes German fund to new investors on subprime credit market woes
AXA Money Market Fund HaircutFRANKFURT (Thomson Financial) – AXA SA’s unit AXA Investment Managers on July 23 closed a 680 mln eur money market fund to new investors after its value slumped 13 pct in one day, Wirtschafts Woche reported, without saying where it got the information.
Some 41 pct of the fund was invested in sub-prime loans, it said.
AXA has decided to offer to buy back shares in the AXA IM US Libor Plus fund, which was offered to investors in Germany, at above market prices to stop a decline in liquidity as investors pulled their money out of the fund, it said.

At a time like this, it probably makes sense to know exactly what your money market fund is invested in.