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Mortgage-backed securities are bonds with cash flows tied to the principal and interest payments on a pool of underlying mortgages. Mortgage securitization has a long history (eg see Goetzmann & Newman, 2010; de Jong et al., 2022), but the birth of the modern US MBS market is typically dated to the issuance of the first agency MBS pool by Ginnie Mae in 1970. Figure 15.1 documents the enormous growth in the MBS market over the past half-century. The figure breaks down the market along two key dimensions.