A grantor trust is a pass-through vehicle that, like a REMIC, issues separately traded classes. However, grantor trusts are treated differently than REMICs for federal income tax purposes, and, unlike a REMIC, the classes in grantor trusts cannot be "time tranched." This means each grantor trust class must receive its proportionate share of principal from the underlying collateral each month until such collateral is paid off. Each holder of a grantor trust certificate is treated for tax purposes as owning an undivided interest in the underlying collateral. The mortgage-related assets serving as collateral and providing the cash flow for Fannie Mae grantor trusts are specifically described in the trust's disclosure documents.