G

Gamete: Mature male or female reproductive cell (sperm or ovum) with a haploid set of chromosomes (23 for humans).

Ganglioside: A particular class of glycosphingolipid present in nerve tissue and in the spleen.

Gastroenterologist: Doctor specializing in diseases of the digestive (gastrointestinal) tract.

Gastrointestinal tract: Digestive tract consisting of the organs and structures that process and prepare food to be used as energy; for example, the stomach, small intestine and large intestine.

Gel Electrophoresis: a DNA separation technique that is very important in DNA sequencing. Standard sequencing procedures involve cloning DNA fragments into special sequencing cloning vectors that carry tiny pieces of DNA. The next step is to determine the base sequence of the tiny fragments by a special procedure that generates a series of even tinier DNA fragments that differ in size by only one base. These nested fragments are separated by gel electrophoresis, in which the DNA pieces are added to a gelatinous solution, allowing the fragments to work their way down through the gel. Smaller pieces move faster and will reach the bottom first. Movement through the gel is hastened by applying an electrical field to the gel.

Gene: The fundamental physical and functional unit of heredity. A gene is an ordered sequence of nucleotides located in a particular position on a particular chromosome that encodes a specific functional product (i.e., a protein or RNA molecule). See gene expression.

Gene expression: The process by which a gene's coded information is converted into the structures present and operating in the cell. Expressed genes include those that are transcribed into mRNA and then translated into protein and those that are transcribed into RNA but not translated into protein (e.g., transfer and ribosomal RNAs).

Gene family: Group of closely related genes that make similar products.

Gene library: See genomic library.

Gene mapping: Determination of the relative positions of genes on a DNA molecule (chromosome or plasmid) and of the distance, in linkage units or physical units, between them.

Gene product: The biochemical material, either RNA or protein, resulting from expression of a gene. The amount of gene product is used to measure how active a gene is; abnormal amounts can be correlated with disease causing alleles.

Gene therapy: New treatment in which defective genes are replaced with normal ones. The new genes are delivered into cells by virus or protein "carriers".

Genetic code: The sequence of nucleotides, coded in triplets (codons) along the mRNA, that determines the sequence of amino acids in protein synthesis. The DNA sequence of a gene can be used to predict the mRNA sequence, and the genetic code can in turn be used to predict the amino acid sequence.

Genetic counseling: Counseling people who may have a gene that makes them more likely to develop cancer. The purpose of the counseling is to explore what the genetic test results might mean, help them decide whether or not they wish to be tested, and to support them before and after the test. 

Genetic engineering technology: See recombinant DNA technology.

Genetic map: See linkage map.

Genetic material: See genome.

Genetics: The study of the patterns of inheritance of specific traits.

Genetic testing: Test to see if a person has certain gene changes known to increase cancer risk. Such tests are recommended only for those with specific types of family history.

Genome: All the genetic material in the chromosomes of a particular organism; its size is generally given as its total number of base pairs.

Genome project: Research and technology development effort aimed at mapping and sequencing some or all of the genome of human beings and other organisms.

Genomic library: A collection of clones made from a set of randomly generated overlapping DNA fragments representing the entire genome of an organism. Compare library, arrayed library.

Genomic sequence: The order of the subunits, called bases, that make up a particular fragment of DNA in a genome. DNA is a long molecule made up of four different kinds of bases, which are abbreviated A, C, T, and G. A DNA fragment that is 10 bases long might have a base sequence of, for example, ATCGTTCCTG. The particular sequence of bases encodes important information in an individual's genetic blueprint, and is unique for each individual (except identical twins).

Germ cell: Reproductive cells, that is, ova (eggs) or sperm.

GI tract: (See gastrointestinal tract.) 

Glands: Cell or group of cells that produce and release substances used nearby or in other parts of the body.

Gleason score: A grading of prostate cancer cells on a scale of 2 to 10. The higher the number, the faster the cancer is likely to grow and spread beyond the prostate.

Glioma: A sarcoma of neurologlial origin.  A neoplasm or tumor composed of neuroglial cells. (Glioma retinae - malignant tumor of the retina (occurs in children) - metastasizes late.

Glioblastoma multiforme (GBM): neoplasm of the central nervous system, especially the cerebellum of the brain, consisting of various cell types.

Glycosphingolipid: Group of carbohydrate-containing fatty acid derivatives of ceramide.  Three classes of these lipids are cerebrosides, gangliosides, and ceramide oligosaccharides.  Absence of enzymes essential to metabolism of these causes glycosphingolipids to accumulate, particularly in the nervous system.  Death is the typical outcome.

Grade: Reflects how abnormal a cancer cell looks under the microscope. There are several grading systems for different types of cancer, such as the Gleason grades for prostate cancer. Each system divides cancer cells into those with the greatest abnormality, the least abnormality, and those in between. Cancers with more abnormal-appearing cells tend to grow and spread more quickly and have a worse prognosis. 
Grading is done by a pathologist who examines tissue from a biopsy.

Graft versus host disease (GVHD): Condition that results when the immune cells of a transplant (usually bone marrow) from a donor attack the tissues of the person receiving the transplant.

Growth factor: Naturally occurring protein that causes cells to grow and divide. Too much growth factor production by some cancer cells helps them grow very quickly. New treatments to block these growth factors are now being tested. Other growth factors help normal cells recover from side effects of chemotherapy.

Guanine (G): A nitrogenous base, one member of the base pair GC (guanine and cytosine).

Gynecologic oncologist: Doctor specializing in cancers of women's reproductive organs. 

Gynecologist: Doctor specializing in women's health.