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In medicine, cancer is a general term for any of a number of different diseases where some of the body's own cells divide in an uncontrolled manner. The resulting new cells can form a malignant tumor (a neoplasm) or propagate throughout the body.

Cancer is, ultimately, a disease of genes. Typically, a series of several mutations is required before a cell becomes a cancer cell. We distinguish between oncogenes, which promote cancer when "switched on" by a mutation, and tumor suppressor genes, which prevent cancer unless "switched off" by a mutation. These mutations can have various causes: radiation or chemicals called carcinogens; some inherited predisposition is not uncommon; some viruses that can cause cancer have also been described. Usually, they carry in their genome some oncogene or tumor suppressor inactivating gene. In about 15% of all cancers, viruses seem to play a role; Bacteria, like Helicobacter pylori, also induce carcinogenesis by a process of chronic inflammation. Finally, damage by free radicals, which are a natural by-product of oxygen metabolism, can cause mutations in the DNA.

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:

  • Immortality. The usual number of cell divisions for a mammalian cell is 50-60 (cell senescence), then it ceases to divide. Tumor cells keep dividing forever.
  • Altered morphology.
  • Building of cellular clusters (Foci).
  • Loss of contact inhibition.
  • Low or no need for growth factors.

Items 2-4 (above) can sometimes be traced to mutations in genes that result in a disruption of cell adhesion. Some cell adhesion proteins are oncogenes.