Group of enzymes that hydrolytically cleave the internucleotide compounds of nucleic acids. They are ubiquitously distributed in the organism and are mainly involved in the catabolism of nucleic acids. Furthermore, they are involved in partial steps of synthesis pathways (DNA replication, DNA repair, RNA processing). The majority of nucleases prefer to cleave single-stranded RNA or DNA. Others prefer double-stranded nucleic acids. Depending on the substrate specificity (DNA or RNA) and the mode of degradation, a distinction is made between deoxyribonucleases, ribonucleases, endonucleases and exonucleases.