Behind Wall Street's subprime fear index

At a time when almost nothing about subprime securities seems certain, the ABX index is a key point of reference for investors navigating the world of risky mortgage debt.

The ABX, launched in January 2007, serves as a benchmark of the market for securities backed by home loans issued to borrowers with weak credit.

"Creating visibility and transparency was a goal of the index, but not everyone knew it would be so closely followed," said Ben Logan, a managing director at Markit Group, a London-based company that specializes in credit derivative pricing and administers the index.

Underlying the mortgage mess has been the fact that "no one knows what subprime securities are really worth," said Barry Ritholtz, director of equity research for New York-based FusionIQ, a quantitative research and asset management firm.