Renin is a protein released by the juxtaglomerular cells of the kidney. It catalyzes the first step in the activation pathway of angiotensinogen - a cascade that can result in aldosterone release, vasoconstriction, and increase in blood pressure. Renin cleaves angiotensinogen to form angiotensin I, which is converted to angiotensin II by angiotensin I converting enzyme, an important regulator of blood pressure and electrolyte balance. Renin occurs in other organs than the kidney, e.g., in the brain, where it is implicated in the regulation of numerous activities.

The renin gene spans 12 kb of DNA and contains 8 introns. The structure of the renin gene is similar to that of pepsinogen, a closely related aspartyl protease. Thus, renin and pepsinogen probably have a common evolutionary origin.