The non-agency MBS market has become the major source of growth in retained mortgage portfolios at Fannie Mae and Freddie Mac in 2004, according to a new Inside MBS & ABS analysis. Through the first eight months of this year, non-agency MBS issuers set aside a total of $146.67 billion of securities classes eligible for purchase by either of the government-sponsored enterprises, or 28.4 percent of total production in the market.