A class of enzymes that degrade DNA or RNA molecules by cleaving the phospho-diester bonds that link adjacent nucleotides. In deoxyribonuclease (DNase), the substrate is DNA. In endonuclease, it cleaves at internal sites in the substrate molecule. Exonuclease progressively cleaves from the end of the substrate molecule. In ribonuclease (RNase), the substrate is RNA. In the S1 nuclease, the substrate is single-stranded DNA or RNA. Nucleases have varying degrees of base-sequence specificity, the most specific being the restriction endonucleases.