Ribonuclease A (RNase A) is an endonuclease that cleaves single-stranded RNA. Bovine pancreatic RNase A is one of the classic model systems of protein science.RNase A is a relatively small protein. It can be characterized as a two-layer α + β protein that is folded in half to resemble a taco, with a deep cleft for binding the RNA substrate. The first layer is composed of three alpha helices (residues 3-13, 24-34 and 50-60) from the N-terminal half of the protein. The second layer consist of three β-hairpins (residues 61-74, 79-104 and 105-124 from the C-terminal half) arranged in two β-sheets. The hairpins 61-74 and 105-124 form a four-stranded, antiparallel β-sheet that lies on helix 3 (residues 50-60). The longest β-hairpin 79-104 mates with a short β-strand (residues 42-45) to form a three-stranded, antiparallel β-sheet that lies on helix 2 .