Nuclease S1 isolated from Aspergillus oryzae exhibits endo-and exolytic hydrolytic activity for the phosphodiester bonds of single-stranded DNA and RNA yielding 5’-phosphomononucleotide and 5’-phosphooligonucleotide end-products. It is used to digest non-annealed polynucleotide tails and hairpin loops in RNA and DNA duplexes and can be used to convert superhelical DNA to the linear form.