A neuroendocrine tumor begins in the hormone-producing cells of the body’s neuroendocrine system, which is made up of cells that are a cross between traditional endocrine cells (or hormone-producing cells) and nerve cells. Neuroendocrine cells are found throughout the body in organs, such as the lungs and gastrointestinal tract (such as the stomach and intestines), and perform specific functions, such as regulating the air and blood flow through the lungs and controlling the speed at which food is moved through the gastrointestinal tract.