J. Edgar Hoover was the first Director of the Federal Bureau of Investigation (FBI). He was originally appointed to head the fledgling FBI in the 1930s by President Franklin Delano Roosevelt. 
Now as we all know, U.S. Presidential Administrations come and go every 4-8 years as do each Administration's respective political appointees. J. Edgar Hoover, however, was an unusual political appointee. He somehow managed to stay on as FBI Director from one administration to the next -- Democrat to Democrat to Republican to Democratic to Democrat to Republican -- clear up to 1972! That doesn't happen every day in U.S. National Politics; actually, it doesn't happen ever, generally.
Now, certainly the fact that Mr. Hoover essentially built the FBI meant a new President could probably argue: "hey, J. Edgar Hoover built "The Bureau" so he probably best new how to run it; don't 'fix' what isn't broken, my father always said."
And of course we out here in the general public could probably also speculate that -- by virtue of his position and administrative resources -- Hoover perhaps was privy to information on each new President and used said information to maintain his position as FBI Director from one Presidential Administration to the next. We might also speculate that perhaps certain (little-discussed) domestic security and espionage threats existed during Mr. Hoover's Cold War-era tenure such that a patriotic FBI director had no other reasonable choice but to keep his position.
It doesn't really matter. But the point is that Mr. Hoover was a Presidential Appointee never displaced by the ebb and flow of the Presidential election cycle. 
Now in light of Hoover, lets examine the work-history of current Defense Secretary, Robert M. Gates over the last few years.
Secretary Gates was appointed by Republican President George W. Bush in 2006 to replace outgoing Secretary of Defense, Donald Rumsfeld. In November 2008, National Elections were held and Democrat Barack Obama was elected as President of the United States.
Naturally, in 2009 the new U.S. President would start to fill his executive branch with political appointees including a new Secretary of Defense. But for the first time in history, the person holding the office of Secretary of Defense -- Robert M. Gates -- didn't get replaced when the Party in control of the White House changed from Republican to Democrat (or vice versa)!
Did Secretary Gates take a few (hypothesized) "cues" from the administration-impervious career of former FBI director J. Edgar Hoover? Was Gates perhaps an "Honor Student" at the little-heralded "J. Edgar Hoover School of Political Permanence?" Or is the military situation currently confronting the U.S. of such a delicate and/or threatening nature to national security that consistent leadership of the Department of Defense is required?
Who knows on all accounts.
