A complimentary DNA for a messenger RNA (mRNA) molecule. Unlike an mRNA, a cDNA can be easily propagated and sequenced.

CentiMorgan (cM)
A unit used to express distances on a genetic map. In genetic mapping, distances between markers are determined by measuring the rate of meoitic recombination between them, which increases proportionately with the distance separating them. A cM is defined as the length of an interval in which there is a 1% probability of recombination. On the average, 1 cM is roughly equivalent to 1 megabase (Mb) of DNA, although this can vary widely due to hot and cold spots of recombination.

CentiRay (cR)
A unit of genetic map distance defined corresponding to an interval in which there is a 1% probability of X-irradiation induced breakage. To be completely specified, the unit must be qualified by the radiation in dosage in rads (e.g. cR8000), because this determines the actual breakage probability.

Expressed sequence tag (EST)
Partial gene sequence data of a cDNA clone which provides a sequence tag for a gene. In order to achieve a very high throughput, these sequences are usually only subjected to a single-pass of sequencing so the error rate in these sequences can be high, perhaps approaching 5%.

The tendency of markers to be inherited together. Linkage of two markers is an indication that they are close to one another in the genome.

Lod Score
A measure of the likelihood of two markers being linked. It is calculated as the log of an odds ratio, so higher values indicate stronger linkage.

A "landmark" that can be localized to a specific region of the genome. All markers on the Human Transcript Map are STSs.

Polymorphic Marker
A marker that displays variability in the population, thereby allowing its inheritance to be followed. A marker must be polymorphic in order to be mapped by genetic means, although any marker can be placed on a physical map.

Radiation Hybrid (RH) Mapping
A method of physical mapping that makes use of the frequencies of X-ray induced DNA breakage to infer distances between markers.

Radiation Hybrid (RH) Panel
A set of DNA samples prepared from a collection of radiation hybrids. Each radiation hybrid is a clonal population of cells derived by fusion of lethally X-irradiated doner cells with rodent cells.

Sequence Tagged Site (STS)
A unique, sequence-defined landmark in the genome that can be detected a specific PCR reaction.

STS-Content Mapping
A physical mapping method in which the presence of STSs on large-insert clones (e.g. YACs) is used to infer linkage and ordering of the sites.

Yeast artificial chromosome (YAC)
A large-insert DNA clone which contains the sequence elements necessary for chromosome function in yeast, together with a fragment of foreign (e.g. human) DNA.