28 U.S. Code § 516 states:
Except as otherwise authorized by law, the conduct of litigation in which the United States, an agency, or officer thereof is a party, or is interested, and securing evidence therefor, is reserved to officers of the Department of Justice, under the direction of the Attorney General.
This seems to imply some exemptions existed at the time it was written or may have been anticipated in the future. Do any such exceptions exist?