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. Since an overactive renin-angiotensin system leads to hypertension, renin is an attractive target for the treatment of this disease. Recombinant mouse renin, expressed as prorenin and after activation by trypsin, can be used for enzyme activity assays.