Structured finance is a sector that transfers risk using complex legal entities like securitization and tranching. Securitization pools debt instruments and issues new securities backed by the pool, while tranching splits securities into different risk levels. Mortgage-backed securities represent claims on cash flows from mortgage loans. Collateralized mortgage obligations (CMOs) issue bonds backed by mortgages and use tranching to create bonds of varying risk levels, like sequential or parallel tranching. The subprime mortgage crisis originated in the late 1990s and became apparent in 2007, caused by risky lending practices and the shadow banking system's involvement in securitization.