T4 RNA Ligase is a ligation enzyme for joining single-stranded RNA or DNA fragments by catalyzing the formation of phosphodiester bonds between single-strandedRNA or DNAcontaining 5"-phosphate and 3"-hydroxyl termini. This enzyme requires ATP for activity, and its activity is highest for RNA-RNA ligation, lower for RNA-DNA ligation, and lowest for DNA-DNA ligation. 3"-end labelingisoften completed using 5"-[32P]pCp.