Renin, a protease secreted by the kidney, plays a key role in the renin-angiotensin system. Renin acts by converting the renin substrate, angiotensinogen, to angiotensin I in the blood, which is further converted into angiotensin II. Angiotensin II is a potent pressor agent causing vasoconstriction. Recombinant human renin, expressed as prorenin and converted to the active enzyme, can be used in FRET-based enzyme activity assays or as a positive control for Western blot, IP, and ELISA.