Mortgage LoanMortgage LoanMortgage debt obligation evidenced, or when made will be evidenced, by the Loan Documents, or  a mortgage debt obligation with a Fannie Mae credit enhancement. refinancing a Portfolio Mortgage LoanPortfolio Mortgage LoanMortgage Loan purchased by Fannie Mae and held as of a certain date regardless of whether it is a Cash Mortgage Loan or an MBS Mortgage Loan. that does not meet TierTierTier 1, Tier 2, Tier 3, or Tier 4 per the Multifamily Underwriting Standards (Form 4660). 2 underwriting requirements.