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A tumor is a biological cell or a group of cells in a multicellular organism that have several distinct properties:

If the tumor is not removed by the immune system, it will develop into cancer.

A cell that degenerates into a tumor cell does usually not acquire all these properties at once, but its daughter cells are selected to build them. This process is called cellular evolution. A first dstep in the development of a tumor cell is usually a small change in the DNA, often a point mutation, which leads, among other things, to a genetic instability of the cell. The instability increases to a point where the cell loses whole choromosomes, or has doubles ones. Also, the DNA methylation pattern od the cell changes, activating and deactivating genes more or less at random. Cells that divide at a high rate, such as stem cells, show a higher risk of becoming tumor cells than those which divide less or not at all, for example, neurons.

In cellular model systems, cells are exposed to carcinogenic influences (chemicals, radiation). In these systems, the first signs of a cell developing into a tumor cell are :

See also : Apoptosis -- Oncogene -- Tumor suppressor gene