The term Special Missions Unit (SMU), at one time referred to as a "Tier 1" unit,[1] is used in the United States to categorize the nation's most highly secretive and elite military special operations forces.[2] The term "special missions unit" is also used in Australia to describe the Special Air Service Regiment. Special mission units have been involved in high-profile military operations, such as the killing of Osama bin Laden and the attempted hostage rescue of Kayla Mueller.