A protein found inside neuroendocrine cells, which release chromogranin A and certain hormones into the blood. Chromogranin A may be found in higher than normal amounts in patients with certain neuroendocrine tumors, small cell lung cancer, prostate cancer, and other conditions. Measuring the amount of chromogranin A in the blood may help to diagnose cancer or other conditions or find out how well treatment is working or if cancer has come back. Chromogranin A is a type of tumor marker. Also called CgA.