... A number of entities that are key players in the U.S. financial system -- including mortgage finance companies Fannie Mae and Freddie Mac, and securities clearinghouses like the Options Clearing Corp Depository Trust Co -- are likely to be downgraded by Standard & Poor's on Monday.

For Fannie Mae and Freddie Mac, losing their triple-A rating could lift borrowing costs, potentially making mortgages more expensive for consumers and adding to stress in the already unstable U.S. housing market. ...

... A downgrade of Fannie Mae and Freddie Mac would affect billions of dollars of debt issued by public housing authorities secured by federally guaranteed mortgages. ...