NEW YORK ( TheStreet) -- Mutual funds that invest in mortgages have been rallying.The big winners include Angel Oak Multi-Strategy Income ( ANGLX), which has returned 20% this year and ranked as the top performing intermediate-term fund, according to Morningstar. Mortgage specialist TCW Total Return Bond ( TGLMX) returned 11.4%, compared to 4.0% for the Barclays Capital Aggregate benchmark. These mutual funds have benefited from a broad rally that has lifted mortgages of all credit qualities. Investors have even embraced subprime mortgages, the toxic assets that helped trigger the financial crisis. As foreclosure rates have fallen lately, prices of some subprime mortgages have climbed to 50 cents on the dollar, up from 40 cents a year ago. High-grade issues jumped recently after the Federal Reserve announced that it would buy $40 billion of mortgages a month as part of the QE3 policy. Mortgage securities represent pools of loans. Investors in the securities collect the principal and interest that borrowers pay. Part of the appeal of mortgages is their relatively attractive yields. At a time when 10-year Treasuries yield only 1.62%, mortgages backed by agencies such as Fannie Mae ( FNMA) yield about 2.1%. Because of their government support, the agency mortgages come with no default risk. To get fatter yields, investors can buy securities that are not backed by government agencies. High-quality nonagency mortgages, known as prime, yield about 4.5%. Risky subprime mortgages can yield 8.5%. Nonagency mortgages were slammed during the financial crisis. With foreclosure rates spiking, some subprime mortgages traded for a few cents on the dollar. Many triple-A-rated securities were downgraded to junk. After markets hit bottom in 2009, mortgages began recovering. Today investors remain wary, and many prime securities trade at 70 cents on the dollar. The prices remain undervalued, argues Brad Friedlander, portfolio manager of Angel Oak Multi-Strategy. "This is one of the cheapest sectors of the fixed-income markets," he says. While Angel Oak has the flexibility to buy many kinds of fixed-income securities, the fund currently has 78% of assets in nonagency mortgages.