A number of produce cancer in a of . Among these are heredity, viruses, ionising radiation, , and in the immune system. For a long time these various seemed to work in different ways, but now are studying how they might in a multifactorial, resulting in malignancy. Basically, cancer is a genetic . Gene can be inherited or they can be in a body cell by a virus, or by damage from an outside . Probably a of mutations leads to a single cell that is malignant and proliferates as a clone. Originally it was thought that a malignant clone was completely , and that the only way to cure cancer was to rid the body of all the cells. now that the problem may be a loss of the ability of the cell to into its , state, perhaps because of the inability to produce a necessary .

Heredity
It is that no more than 20 per cent of cancers are based on inheritance. Several types of cancer, however, do run in families. Breast cancer is an example. Cancer of the colon is more common in families that tend towards polyps in the colon. A type of retinoblastoma has been to only when a gene is deleted. These genes, called tumour-suppressor genes or antioncogenes, act to prevent cellular replication. Their deletion prevents the check on multiplication of cells, that is, " the brakes". In some hereditary disorders, the chromosomes a high frequency of breakage; such diseases carry a high risk of cancer.

Viral
Viruses are the cause of many cancers in animals. In humans the Epstein-Barr virus is associated with Burkitt's lymphoma and lymphoepitheliomas, the hepatitis virus with hepatocarcinoma, and a papilloma virus with carcinoma of the cervix. These viruses associated with human tumours are DNA viruses. The HTLV virus that produces a T-cell leukaemia is an RNA virus, or retrovirus, as are most of the viruses associated with animal tumours. In the presence of an enzyme called transcriptase, they the infected cell to make DNA copies of the virus's genes, which can be into the cell genome (the full of DNA). Such viruses may contain a gene, called a viral oncogene, of cells into malignant ones. that each viral oncogene has a counterpart in the normal human cell called a proto-oncogene, or cellular oncogene. Oncogene gene products (i.e., proteins for which they ) have been as growth (or proteins necessary for the action of growth ), which stimulate the growth of tumour cells.

Radiation
Ionising radiation is a potent cause of cancer. Radiation changes in DNA, including chromosome breaks and transpositions, in which the broken-off ends of two chromosomes are exchanged. It acts as an of carcinogenesis, a change that progresses to cancer after a latent of years. This delay provides opportunity for to other .