M

Macrophage: Type of white blood cell that engulfs and destroys foreign materials.

Macrorestriction map: Map depicting the order of and distance between sites at which restriction enzymes cleave chromosomes.

Magnetic resonance imaging: Taking pictures of the inside of the body using a powerful magnet (instead of X-rays) and radio waves. Images appear on a computer screen as well as on film. Some find the painless procedure psychologically uncomfortable because they are confined inside the MRI machine during the examination.

Malignant tumor: A mass of cancer cells capable of or having invaded surrounding tissues or spreading (metastasize) to distant areas of the body.

Mammogram, mammography: Breast X-ray capable of finding breast cancer that cannot be felt. Special x-ray machines are used only for this purpose. Diagnostic mammography helps the doctor learn more about breast masses or the cause of other breast symptoms.

Mapping: See gene mapping, linkage map, physical map.

Margin : The edge of tissue(s) removed during surgery. A "negative" margin is a sign no cancer was left behind. A "positive" margin shows cancer cells are at the edge of the removed tissue and is usually a sign that some cancer was left in the body.

Marker: An identifiable physical location on a chromosome (e.g., restriction enzyme cutting site, gene) whose inheritance can be monitored. Markers can be expressed regions of DNA (genes) or some segment of DNA with no known coding function but whose pattern of inheritance can be determined. See restriction fragment length polymorphism.

Mastectomy: Surgical removal of all or part of the breast and sometimes other tissue.
-- Extended radical mastectomy: Removal of the breast, skin, nipple, areola, chest muscles (pectoral major and minor), and all axillary and internal mammary lymph nodes on the same side.
-- Halsted radical mastectomy: Removal of the breast, skin, nipple, areola, both pectoral muscles, and all axillary lymph nodes on the same side.
-- Modified radical mastectomy: Removal of the breast, skin, nipple, areola, and most of the axillary lymph nodes on the same side, leaving the chest muscles intact.
-- Partial mastectomy: Removal of less than the whole breast, taking only part of the breast in which the cancer occurs and a margin of healthy breast tissue surrounding the tumor.
-- Prophylactic mastectomy: A subcutaneous mastectomy done before any evidence of cancer can be found, for the purpose of preventing cancer. (Sometimes recommended for women at very high risk of breast cancer.)
-- Quadrantectomy: Partial mastectomy in which the quarter of the breast that contains a tumor is removed.
Segmental mastectomy is a partial mastectomy.
-- Simple mastectomy or total mastectomy: Removal only of the breast and areola.
-- Subcutaneous mastectomy: Removal of the internal breast tissue. The nipple and skin are left intact. 

Mb: See megabase.

Mediastinoscopy: Chest cavity exam using a lighted tube inserted under the chest bone (sternum) that permits the doctor to see lymph nodes in this area and to remove samples to check for cancer.

Medical oncologist: Doctor who specializes in diagnosing and treating cancer with chemotherapy and other drugs.

Megabase (Mb): Unit of length for DNA fragments equal to 1 million nucleotides and roughly equal to 1 cM.

Meiosis: The process of two consecutive cell divisions in the diploid progenitors of sex cells. Meiosis results in four rather than two daughter cells, each with a haploid set of chromosomes.

Melanoma: Cancerous (malignant) tumor that begins in the cells that produce skin color (melanocytes). Melanoma is almost always curable in early stages. However, it is likely to spread to other parts of the body and then chances for a cure are greatly reduced.

Menarche: A woman's first menstrual period. Early menarche (before age 12) is a risk factor for breast cancer -- possibly because the earlier a woman's periods begin, the longer she is exposed to estrogen. (See: Estrogen.)

Menopause: When a woman's monthly menstruation cease forever and the level of hormones produced by the ovaries decrease -- usually occurs in the late 40s or early 50s, but can also be brought about by surgical removal of both ovaries (oophorectomy), or by some chemotherapies that destroy ovarian function.

Messenger RNA (mRNA): RNA that serves as a template for protein synthesis. The molecule that carries information from the DNA genetic code to areas in the cytoplasm of a cell that make proteins.

Metastasis (Mestastasize): Spread of cancer cells to distant parts of the body via the lymph system or bloodstream.

Metaphase: A stage in mitosis or meiosis during which the chromosomes are aligned along the equatorial plane of the cell.

Metastasis: Migration of cells, especially cancer cells, from on part of the body to another. The spread is carried out by the lymphatic system or the blood stream.

Metastasize: To invade by metastasis.

Micrometastases: The spread of cancer cells in groups so small that they can only be seen under a microscope.

Mitosis: The process of nuclear division in cells that produces daughter cells that are genetically identical to each other and to the parent cell.

Modified radical mastectomy: Removal of the breast, skin, nipple, areola, and most of the axillary lymph nodes on the same side, leaving the chest muscles intact.

Monoclonal antibodies: Antibodies made in the lab that are designed to target specific substances called "antigens". Monoclonal antibodies which have been attached to chemotherapy drugs or radioactive substances are being studied to see if they can seek out antigens that are unique to cancer cells and deliver these treatments directly -- and ONLY -- to the cancer, killing cancer cells without harming healthy tissue. Monoclonal antibodies also are used to help find and classify cancer cells.

Morbidity: Measure of the new cases of a disease in a population; the number of people who have a certain disease.

Mortality: Measure of the rate of death from a disease within a given population.

MRI: (See: Magnetic resonance imaging.)

mRNA: (See: Messenger RNA.)

Mucinous carcinoma: Type of carcinoma that is formed by mucus-producing cancer cells.

Mucositis: Inflammation of mucous membrane.

Multidrug resistance (MDR): Tumor cell resistance to several unrelated drugs after exposure to a single chemotherapy drug.

Multifactorial or multigenic disorder: See polygenic disorder.

Multiplexing: Sequencing approach that uses several pooled samples simultaneously, greatly increasing sequencing speed.

Mutation: A change; a change in a gene. Any heritable change in DNA sequence. (Compare to polymorphism.)

Myoclonic: Intermittent tensing and relaxing of muscles.