Uridine triphosphate, labeled on the alpha phosphate group with ³⁵S, is typically used in reactions involving enzymes that will incorporate the ribonucleoside monophosphate (base, sugar, and alpha-labeled phosphate) into a chain of RNA. The advantage of ³⁵S, when compared to other short-lived isotopes, is its relatively low signal (which contributes to a sharp bａnd width), as well as longer shelf life.

Common applications include:

• RNA labeling
• Labeling with SP6 RNA polymerase, T3 RNA polymerase, or T7 RNA polymerase