Chemicals (e.g., from smoking), radiation, viruses, and heredity all contribute to the development of cancer by triggering changes in a cell's genes. Chemicals and radiation act by damaging genes, viruses introduce their own genes into cells, and heredity passes on alterations in genes that make a person more susceptible to cancer. Genes are inherited instructions that reside within a person's chromosomes. Each gene instructs a cell how to build a specific product--in most cases, a particular kind of protein. Genes are altered, or "mutated," in various ways as part of the mechanism by which cancer arises.
![Genes and Cancer](https://webarchive.library.unt.edu/eot2008/20090114175911im_/http://www.cancer.gov/images/Documents/4167b7ca-7e27-4eec-9855-640637dde5dc/cancer39.jpg)
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