FAS 157, a fair value accounting rule, requires banks to mark assets to their current market value, even if the market is illiquid. Some argue this is forcing banks like Citigroup and Merrill Lynch to overstate losses on investments like CDOs backed by subprime mortgages. However, others counter that FAS 157 provides transparency and that the losses are real, not just on paper. While intended to make markets more efficient, some believe FAS 157 may be exacerbating problems in the financial system during the current credit crisis.