Fannie Mae and Freddie Mac combined have consistently been the largest players in the market, owning or guaranteeing about half or more of the mortgages in the sample at any given time. Non-agency securitization peaked in the first quarter of 2006, when it accounted for nearly 40% of new originations. Finally, the share of mortgages retained in the originating institution’s portfolio averaged about 15% throughout the boom, but has fallen considerably since. (from SF Fed via Calculated Risk)