S1 nuclease

S1 nuclease is an endonuclease that is active against single-stranded DNA and RNA molecules. Its reaction products are oligonucleotides or single nucleotides with 5' phosphoryl groups. Although its primary substrate is single-stranded, it can also occasionally introduce single-stranded breaks in double-stranded DNA or RNA, or DNA-RNA hybrids. It is used in the laboratory as a reagent in nuclease protection assays. In Molecular biology it is used in removing single stranded tails from DNA molecules to create blunt ended molecules and opening hairpin loops generated during synthesis of double stranded cDNA.