From 2003 to 2006, new issues of CDOs backed by asset-backed and mortgage-backed securities had increasing exposure to subprime mortgage bonds. Mezzanine ABS CDOs are mainly backed by the BBB or lower-rated tranches of mortgage bonds, and in 2006, $200 billion in mezzanine ABS CDOs were issued with an average exposure to subprime bonds of 70%.[citation needed] As delinquencies and defaults on subprime mortgages occur, CDOs backed by significant mezzanine subprime collateral experience severe rating downgrades and possibly future losses.