Cabinet Departments (the 15 "Cabinet" heads of these Departments advise the president in their area--such as foreign policy or transportation--and can be fired by the president at any time. Independent Regulatory Agencies, such as the FCC, protect the public and although the heads are chosen by the president once they are in office they cannot be fired. Government Corporations--U.S. Postal Service is the largest--provide services for a fee. Independent Executive Agencies, such as the FDA or EPA or NASA, protect the public and the heads can be fired by the president at any time.