An enzyme that causes splitting of ribonucleic acid. Pancreatic ribonuclease for example, cleaves only phosphodiester bonds that are linked to pyrimidine-3′-phosphates. It is a critical regulator of life processes in the cell. The first enzyme to be synthesized (1969), it is composed of 124 amino acid residues. It is one of the proteins for which the sequence of amino acids has been elucidated (the order or sequence of amino acids is of critical importance in the functioning of enzymes, genes, and nucleotides).See Genetic Code; RNase.