If securities held in a mortgage backed security are prepaid, then a decrease in the assets will cause a portfolio runoff. These types of mortgage backed portfolios face pre-payment risk that can often cause the fund to reinvest the proceeds at lower yields than originally anticipated.
Related information about portfolio runoff:
- Portfolio Runoff Definition | Investopedia
 A decrease in the assets of a mortgage-backed securities portfolio due to the   prepayment of the securities held in that portfolio. It is risk these portfolios face, ...
 
- Portfolio Runoff - Financial Dictionary - The Free Dictionary
 A reduction in the mortgages represented in a mortgage-backed security as a   result of prepayment of some of the mortgages in it. This can reduce the yield on ...
 
- What is portfolio runoff? definition and meaning - InvestorWords.com
 Definition of portfolio runoff: If securities held in a mortgage backed security are   prepaid, then a decrease in the assets will cause a portfolio runoff. These types ...
 
- What is portfolio runoff? - BusinessDictionary.com
 Definition of portfolio runoff: Reinsuring a portfolio until such time as all of the   ceded premiums have been paid.
 
- portfolio runoff - Insurance Glossary
 portfolio runoff - A practice under which a reinsurance portfolio is allowed to   continue until all ceded premium is earned or all losses are closed, or both.
 
- Portfolio Runoff: Definition from Answers.com
 Portfolio Runoff Process of the continual reinsurance of a ceding company s   portfolio of insurance policies.
 
- Glossary of Reinsurance Terms - Guy Carpenter
 Portfolio Runoff. Continuing to reinsure a portfolio until all ceded premium is   earned, or all losses are settled, or both. While a loss runoff is usually unlimited   as ...
 
- Sr. Manager - Discover Student Loans Credit Risk Portfolio Strategy ...
 Nov 1, 2012 ... Manage loan portfolio to desired profitability through strategy development to   prevent portfolio runoff and to reduce losses. Supporting activities ...