Behind Wall Street's subprime fear index

The part of the ABX linked to the riskiest subprime bonds has fallen about 67 percent since the mortgage wipeout began in the summer. Now subprime fears are spreading to segments of the market once considered ultra-safe.

For example, AAA-rated mortgage-backed debt has tumbled on the ABX in the last month, reflecting concerns that even those bond holders higher up in the capital structure - those who get paid first - may also suffer losses.

Citigroup CEO Gary Crittenden referred to the decline in the AAA index when the financial services giant announced on Nov. 5 it would write down as much as $11 billion in the fourth quarter.

"Now the best way to kind of get an outside perspective on this is to look at the ABX indices, which have dropped dramatically since the end of September," Crittenden said.