This testimony analyzes the transition proposed by S. 1217, the Housing Finance Reform and Taxpayer Protection Act of 2013, proposed by Sens. Corker (R-TN) and Warner (D-VA), and identifies six issues that need to be addressed for a successful transition: (1) the development of a common securitization platform and single security; (2) achieving liquidity for the new MBS contemplated by Corker-Warner; (3) responsibly reducing conforming loan limits; (4) ensuring the continued flow of mortgage finance for underserved market segments; (5) attracting sufficient and appropriately priced capital into the new system; and (6) maintaining sources of countercyclical liquidity. Such a transition would be the largest such undertaking in history, and one that, to the best of my knowledge, has no close precedents. Fannie and Freddie currently hold slightly more than $5 trillion in mortgage-related assets. Since the sudden and steep decline in private mortgage finance that occurred in 2008, the two enterprises have been responsible for more than 60% of the new mortgage originations, about $1.7 trillion each year, an amount equivalent to slightly more than 10% of our nation’s annual gross domestic product. The federal government has some experience in resolving failed institutions — recently, the government’s interactions with AIG and General Motors come to mind, and before that, we had the experience of the Resolution Trust Company in resolving hundreds of failed thrifts. But I can think of no instance in which we have tried to simultaneously resolve large failed institutions and transition their core economic functions into a newly created set of institutions, certainly not on the scale imagined by Corker-Warner. The guiding principle for legislators and regulators who are structuring our housing finance transition must first and foremost be, “Do no harm.” Avoiding the disruption of mortgage liquidity, either systemwide or in individual market segments, should be a paramount concern during this period. A failure to adhere to this principle would be catastrophic for the housing markets and the broader economy.I suggest a number of changes to Corker-Warner's transition plan: (1) delegate more responsibility to regulators and remove arbitrary timetables; (2) phase in the transition in parts (such as starting with the 15-year fixed-rate mortgage or high cost conforming loans), rather than all at once; (3) convert legacy securities into the new MBS created under Corker-Warner; (4) pre-approve these new MBS for use in the To Be Announced Market and as collateral for Fed lending, repo markets and derivatives transactions to increase liquidity; (5) give a running start to institutions focused on underserved markets (particularly affordable rental housing); and (6) prove expanded emergency powers to create sources of countercyclical liquidity.