BRCA1: A gene that normally acts to restrain the growth of cells in the breast but which, when mutated, predisposes to breast cancer . The gene's full name is breast cancer 1, early onset .

The BRCA1 gene belongs to a class of genes known as tumor suppressor genes . Like other tumor suppressor genes, BRCA1 regulates the cycle of cell division by keeping cells from growing and dividing too rapidly or in an uncontrolled way. In particular, it inhibits the growth of cells that line the milk ducts in the breast.

More than 600 mutations in the BRCA1 gene are known. Many are associated with an increased risk of cancer. These mutations can be changes in one or a small number of DNA base pairs, or, in some cases, large rearrangements of DNA. A mutated BRCA1 gene usually makes a protein that does not function properly because it is abnormally short. The defective BRCA1 protein is unable to help correct mutations that occur in other genes. These defects accumulate and may allow cells to grow and divide uncontrollably to form a tumor.

The protein made by the BRCA1 gene is directly involved in the repair of damaged DNA. In the nucleus of many types of normal cells, the BRCA1 protein interacts with the protein produced by the RAD51 gene to mend breaks in DNA. These breaks can be caused by environmental agents such as radiation, but they can also occur naturally when chromosomes exchange genetic material. BRCA2 , which has a function similar to that of BRCA1, also interacts with RAD51. By repairing DNA, these three proteins play a role in maintaining the stability of the human genome .

BRCA1 and BRCA2 were the first breast cancer genes to be identified. Mutated forms of these genes are believed to be responsible for about half the cases of inherited breast cancer, especially those that occur in younger women. In addition to breast cancer, mutations in the BRCA1 gene also increase the risk of ovarian cancer , prostate cancer , and colon cancer .

The BRCA1 gene is located on chromosome 17q21 -- the long (q) arm of chromosome 17 at position 21. The symbol BRCA stands for BReast CAncer.

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