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1. In mortgage-backed securities, the estimated rate at which homeowners will pay off the underlying mortgages of an MBS. The equivalent of the coupon on a mortgage-backed security is a percentage of the interest and principal paid on the mortgages backing the security. A risk associated with mortgage-backed securities is that too many homeowners will pay off their mortgages with too much speed, depriving the holder of the MBS from future interest payments.