The Neighborhood Assistance Corporation of America, a Boston-based nonprofit lender, is making waves with its vocal advocacy in favor of reviving a more sustainable version of the subprime mortgage market. The organization's mission is to expand homeownership with mortgages to lower-income borrowers on terms that are both profitable and sustainable. It is half lender, half community organization. Applicants are subject to careful screening and, upon approval, are required to participate in the organization's advocacy efforts. The lender also replaces the typical mortgage insurance with a requirement that borrowers contribute to an emergency fund. This fund is lent out to borrowers facing medical emergencies or a job loss, preventing defaults. The organization's default rate has been low; of the 4,005 loans it made from 2004 to 2006, the peak of the housing boom, only 2.6 percent lost their homes to foreclosure--less than a third the national rate.