Giannini sought to build a national bank, expanding into most of the western states as well as into the insurance industry, under the aegis of his holding company, Transamerica Corporation. In 1953 regulators succeeded in forcing the separation of Transamerica Corporation and Bank of America under the Clayton Antitrust Act.[8] The passage of the Bank Holding Company Act of 1956 prohibited banks from owning non-banking subsidiaries such as insurance companies.