Fannie Mae CEO Timothy Mayopoulos has provided new details on regulators’ plans to boost mortgage lending, expanding on Federal Housing Finance Agency Director Melvin Watt’s comments that Fannie and Freddie would back loans with down payments as low as 3 percent of the home's value. Mayopoulos expects Fannie’s low-down-payment mortgages to cost the borrower less than Federal Housing Administration loans. But he also said Fannie’s loans would require private mortgage insurance on top of the down payment, which could limit the size of the program. It is not clear whether private mortgage insurers will want to take on the riskier loans. The program could also introduce higher default rates, since the 20 percent down payment has previously acted to filter applicants who were likely to default.