This 5´ adenylated, 3´ blocked oligodeoxynucleotide can be used for cloning short RNAs according to the procedure of Bartel

RNA ligase recognizes the "activated" adenylated oligo and covalently ligates its 5´ end to the 3´ OH of a second single stranded sequence in the absence of ATP. In a mixture of nucleic acids use of the 5´ adenylated, 3´ blocked oligo with RNA ligase (w/o ATP) results in ligation of the target oligo only.