The 121st Support Battalion Distinctive Unit Insignia—also referred to as a DUI or a “unit crest”—was approved on 23 June 2008. Buff and scarlet, two of the colors used in the lower portion of the insignia, are traditionally associated with Support units, while the gold band is sing of excellence. In the chief (upper third) are stylized black peaks representing the Franklin Mountains, a range near El Paso that alludes to the location where the Battalion was organized; mountains are longtime symbols of strength, continuity, constancy, and eternity.
The five-pointed white star simulates a star actually erected on one of the mountain’s southern slopes, while also alluding to the Lone Star State of Texas, home of the organization. A blue band at the foot of the mountains is a visual reference to the Rio Grande River; rivers are sources of support for life in all terrains, making it an appropriate symbol of the role Logisticians play in supporting combat forces in even the most inhospitable territories. Two crossed hammers further highlight the Battalion’s logistics function, and the mechanized track recalls modern military forces. "Utmost In Honorable Service" describe the Battalion's modus operandi when providing its Logistics expertise for the units it supports.
Distinctive Unit Insignias are worn by all Soldiers (except General Officers) in units that have been authorized to be issued the device. It is worn centered on the shoulder loops of the Army Green Service Uniform (AGSU) and the blue Army Service Uniform (ASU, Enlisted only) with the base of the insignia toward the outside shoulder seam. DUIs are not worn on the Dress variations of either uniform, however.
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For most of the first decade of the 21st Century, the 121st Support Battalion—redesignated as the 121st Brigade Support Battalion—served as the Support component of the 4th Heavy Brigade Combat Team until it was reflagged and redesignated as the 123rd Brigade Support Battalion in 2010.