The definitions used in this glossary of terminology either have been provided by the authors of the articles, or have been extracted wholly or in part, or paraphrased from the following sources: The American Medical Association Encyclopedia of Medicine, Charles B. Clayman, MD, Medical Editor, Random House, New York, 1989; Biotechnology from A to Z, 2d Edition, William Bains, Oxford University Press, New York, New York, 2002; A Dictionary of Genetics, 6th Edition, Robert C. King and William D. Stansfield, Oxford University Press, New York, New York, 2002; Dorland's Illustrated Medical Dictionary, 29th and 30th Editions, W. B. Saunders Company, Philadelphia, 2000, 2003; Genes VII, Benjamin Lewin, Oxford University Press, New York, New York, 2000; The Gale Encyclopedia of Genetic Disorders, Volumes I and II, Stacey L. Blachford, Ed., Thomson Learning, New York, New York, 2002; The Merriam-Webster Dictionary, Merriam-Webster, Inc., Springfield, Massachusetts, 1997; Molecular Biology of the Cell, 3rd Edition, Bruce Alberts, et al., Garland Publishing, 1994; The Random House Dictionary of the English Language, Unabridged Edition, 1966; Webster's Ninth New Collegiate Dictionary, 1991.


1. An individual who harbors the specific organisms of a disease without manifest symptoms and is capable of transmitting the infection; the condition of such an individual is referred to as the carrier state.

2. Electron carrier; a chemical substance that can accept one or more electrons and then donate them to another substance (being reduced and then reoxidized).

3. An instrument or apparatus for carrying something.

4. In genetics, an individual who is heterozygous for a recessive gene and thus does not express the recessive phenotype but can transmit it to offspring. Only females can be carriers of X-linked recessive traits.

5. A substance that carries a radioisotopic or other label, as in a tracer study. A second isotope mixed with a particular isotope is also referred to as a carrier.

6. A transport protein that carries specific substances, e.g., in the blood or across cell membranes.

7. In immunology, a macromolecular substance to which a hapten is coupled in order to produce an immune response against the hapten, immune responses being usually produced only against large molecules capable of simultaneously binding both B cells and helper T cells.