Before the subprime crisis, the largest mortgage originators sold their mortgages as MBS either through private labels or with the help of the US government through . In the case of private label RMBS, the issuer would use or credit enhancement to create a AAA Otherwise, that piece of the MBS would not be rated AAA and would find it less desirable. Credit enhancement through a senior/sub structure was more popular in the years leading up to the crisis, which meant there were many low-rated securities available to include in a CDO2. The hedge funds in Michael Lewis' book, , made money betting against the CDOs and CDO2s by buying .

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