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Rous sarcoma virus: Chicken retrovirus that was the first virus shown to cause a malignancy.

sarcoma: Malignant tumor that begins in connective and supportive tissue.

screening: Checking for disease when there are no symptoms.

secondary tumor: Metastasis.

SEER Program: Surveillance, Epidemiology, and End Results Program of the National Cancer Institute. Started in 1973, SEER collects cancer incidence data in nine geographic areas with a combined population of approximately 9.6 percent of the total population of the United States.

side effect: Problem that occurs when treatment affects healthy cells. Common side effects of cancer treatment are fatigue, nausea, vomiting, decreased blood cell counts, hair loss, and mouth sores.

somatic cell: Any of the body cells except the reproductive cells.

SPF (sun protection factor): Scale for rating sunscreens. Sunscreens with an SPF of 15 or higher provide the best protection from the sun's harmful rays.

squamous cell cancer: Type of skin cancer that arises from the squamous cells.

stage: Extent of a cancer, especially whether the disease has spread from the original site to other parts of the body.

staging: Doing exams and tests to learn the extent of the cancer, especially whether it has spread from its original site to other parts of the body.

stem cells: Cells from which all blood cells develop.

sun protection factor: See SPF.

sunscreen: Substance that blocks the effect of the sun's harmful rays. Using lotions or creams that contain sunscreens can protect the skin from damage that may lead to cancer. See also SPF.

survival rate: Proportion of patients alive at some point after their diagnosis of a disease. telomerase: Enzyme that is present and active in cells that can divide without apparent limit (for example, cancer cells and cells of the germ line). Telomerase replaces the missing repeated sequences of each telomere.

telomere: End of a chromosome. In vertebrate cells, each telomere consists of thousands of copies of the same DNA sequence, repeated again and again. Telomeres become shorter each time a cell divides; when one or more telomeres reaches a minimum length, cell division stops. This mechanism limits the number of times a cell can divide.

testosterone: Male sex hormone.

transformation: Change that a normal cell undergoes as it becomes malignant.

tumor: Abnormal mass of tissue that results from excessive cell division. Tumors perform no useful body function. They may be either benign (not cancerous) or malignant (cancerous).

tumor marker: Substance in blood or other body fluids that may suggest that a person has cancer.

tumor suppressor gene: Gene in the body that can suppress or block the development of cancer.

ultraviolet (UV) radiation: Invisible rays that are part of the energy that comes from the sun. UV radiation can burn the skin and cause melanoma and other types of skin cancer. UV radiation that reaches the earth's surface is made up of two types of rays, UVA and UVB rays. UVB rays are more likely than UVA rays to cause sunburn, but UVA rays pass further into the skin. Scientists have long thought that UVB radiation can cause melanoma and other types of skin cancer. They now think that UVA radiation also may add to skin damage that can lead to cancer. For this reason, skin specialists recommend that people use sunscreens that block or absorb both kinds of UV radiation.

X-chromosome inactivation: Process by which one of the two X chromosomes in each cell from a female mammal becomes condensed and inactive. This process ensures that most genes on the X chromosome are expressed to the same extent in both males and females.

X-ray: High-energy radiation used in low doses to diagnose diseases and in high doses to treat cancer.

xeroderma pigmentosum: Hereditary disease characterized by extreme sensitivity to the sun and a tendency to develop skin cancers. Caused by inadequate DNA repair.

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