Angiogenin (Ang) also known as ribonuclease 5 is a protein that in humans is encoded by the ANG gene. Angiogenin is a potent stimulator of new blood vessel formation. It hydrolyzes cellular tRNAs resulting in decreased protein synthesis and is similar to pancreatic ribonuclease.

Angiogenin is a small protein that is implicated in angiogenesis (formation of new blood vessels) in tumor growth. However, angiogenin is unique among the many proteins that are involved in angiogenesis in that it is also an enzyme with an amino acid sequence 33% identical to that of bovine pancreatic ribonuclease (RNase) A. Moreover, although Ang has the same general catalytic properties as RNase A – it cleaves preferentially on the 3' side of pyrimidines and follows a transphosphorylation/hydrolysis mechanism – its activity differs markedly both in magnitude and in specificity.

Although angiogenin contains counterparts for the key catalytic residues of bovine pancreatic RNase A, it cleaves standard RNA substrates 10⁵–10⁶ times less efficiently than does RNase A. Despite this apparent weakness, the enzymatic activity of Ang appears to be essential for biological activity: replacements of important active site residues invariably diminish ribonuclease and angiogenesis activities in parallel, and a substitution that increases enzymatic activity also enhances angiogenic potency.