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Securities backed by a mortgage pool. There are two basic types: pass-through securities, in which the principal and interest paid by the holders of the mortgages are distributed to investors through a government agency or other intermediary; and collateralised mortgage obligations (CMOs), in which the cash flow from the pool is divided among several classes of bonds with different interest rates and maturity dates. 
This is a type of asset-backed security that uses a single mortgage, or a pool of them, as collateral. Investors receive payments derived from the interest and principal of the underlying mortgages.