Serum cholinesterase, often called pseudocholinesterase (PCHE), is distinguished from acetylcholinesterase or true cholinesterase by both location and substrate. Acetylcholinesterase is found in erythrocytes, in the lungs and spleen, in nerve endings, and in the gray matter of the brain. It is responsible for the hydrolysis of acetylcholine released at the nerve endings to mediate transmission of the neural impulse across the synapse. PCHE, the serum enzyme, is also found in the liver, the pancreas, the heart, and in the white matter of the brain. Its biological role is unknown. Low values of PCHE are also found in patients with liver disease. In general, patients with advanced cirrhosis and carcinoma with metastases will show a 50% to 70% decrease. Essentially normal values are seen in chronic hepatitis, mild cirrhosis, and obstructive jaundice.