An enzyme that specifically degrades RNA or single-stranded DNA to 5´ mononucleotides. Purified from the filamentous fungus Aspergillus oryzae, S₁ nuclease is used in assessing the extent of a hybridization reaction by removing unpaired regions. It is also used to remove the sticky ends of restriction fragments. In S₁ mapping, the coding region of a gene is detected by performing mRNA-DNA hybridization and removing unpaired DNA with S₁ nuclease.