Moody's Investor Service has placed the ratings assigned to four of the six largest US bank holding companies on review for a downgrade. The other two banks are also on review, but the ratings direction is uncertain - Moody's is considering the potentially offsetting influence of improvements in the standalone credit profile of their operating subsidiaries. The ratings on the banks' subsidiaries were simultaneously placed on review for an upgrade.Moody's is considering reducing its government support assumptions so as to reflect the impact of US bank resolution policies.Over the past year, Moody's has observed the progress being made towards establishing a framework to resolve any problems with large systemically important banks. This has involved greater cooperation and discussion among international banking regulators to manage the coordinated resolution of any problems regarding global banking groups should they occur.Two opposing effects influence the risks for bond-holders at the holding company level. While the risk of default increases when authorities are less willing to support a failing bank, the severity of any loss, given a default, is likely to reduce with an orderly resolution.The Goldman Sachs Group Inc (A3), JP Morgan Chase & Co (A2), Morgan Stanley (Baa1) and Wells Fargo and Company Inc (A2) are on review for downgrade. Bank of America Corporation (Baa2) and Citigroup Inc (Baa2) are on review, direction uncertain.The bank financial strength ratings on Bank of America N.A. (D+) and Citibank N.A. (D+) were placed on review for upgrade.