But, in fact, in the early 2000s, when the subprime boom unfolded, the industry had changed again. Securitization was more dominant than ever. The GSEs were still responsible for buying and securitizing the top-tier conforming mortgages. But as a new range of actors entered the mortgage market with a more dynamic and expansive agenda, their principal business model was not to disaggregate and to spread the risk but to integrate every step of the process, including the holding of large quantities of securities on their own balance sheets.25 It was this growth model, based on integration, not
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