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By Mayo Clinic staffCancer occurs when a cell develops a mutation in its DNA. The mutation allows the cell to continue growing and dividing when healthy cells would normally die. In the case of carcinoid tumors, the accumulating cells form a mass. Cancer cells can invade nearby healthy tissue and spread to other parts of the body.
It's not clear what causes the mutations that can lead to carcinoid tumors.
Carcinoid tumors develop from cells in the neuroendocrine system, a part of your body that has many functions, including production of certain hormones. These cells also have characteristics similar to nerve cells.