When American house prices began to fall 2007, many "subprime" borrowers, defined as those with poor credit rating and consequently a high risk of default, stopped paying their mortgages, as their debt was greater than the value of their house. Unfortunately, the institutions which had issued the mortgage had created financial products called mortgage-backed securities [MBS] and collateralized debt obligations [CDO], which had been bought by many financial institutions including investment banks, hedge funds, and so on. This process is called securitization: financial assets like mortgages which produce a cash flow are pooled [grouped together] and converted into securities that are then sold to investors.