The Fed is buying about 85 billion dollars in treasury and mortgage back securities a month.

美联储每月购买约850亿美元的公债和抵押贷款支持债券。

Fannie and Freddie maintained a huge mortgage securities market.

房利美和房地美维持着庞大的抵押贷款证券市场。

The industry sprang up in the 1970s, first focusing on municipal debt and later branching out into structured products like mortgage securities.

这个行业在20世纪70年代兴起，起初它们专注于市政债券，然后拓展到结构性产品，比如抵押贷款证券。