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Causes
It is usually impossible to determine the cause of a brain tumor in any given individual. A brain tumor begins when certain genes on the chromosomes of a cell no longer function properly. These genes normally regulate how often the cell divides (or if it divides at all) and repair other genes. Among the genes they normally fix if they're defective: those that tell a damaged cell to self-destruct.
Some people might be born with partial defects in one or more of these genes, and environmental factors (such as radiation and chemicals) might lead to further damage. In other people, environmental factors might be the only cause of damage to the gene function.
Once a cell is dividing too rapidly and can't check its own growth, it can grow into a tumor. If a tumor cell begins to grow, the body's immune system should detect the abnormal cell and kill it. Many tumors, however, produce substances that block the immune system from recognizing the abnormal tumor cells. Over time, the tumor cells overpower all of the body's defenses against their growth.
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