Animals (or embryos) composed of cells of different genetic origin.


Defines both molecular clones and whole-animal clones.


The propagation of genetically exact duplicates of an organism by a means other than sexual reproduction; for example, the vegetative production of new plants or the propagation of DNA molecules by insertion into plasmids. Often, but inaccurately, used to refer to the propagation of animals by nuclear transfer.


Growth of cultured cells together.


Living on or within another organism, and deriving benefit without harming the host.

Control elements

DNA sequences in genes that interact with regulatory proteins (such as transcription factors) to determine the rate and timing of expression of the genes as well as the beginning and end of the transcript.

CpG methylation

A heritable chemical modification of DNA (replacement of cytosine by 5-methyl cytosine) that, when present in a control region, usually suppresses expression of the corresponding gene.

CJD or Creutzfeldt-Jakob Disease

A disease of humans hypothesized to be caused by a prion, or small protein, which alters the structure of a normal brain protein, resulting in destruction of brain neural tissue. The most common form is thought to have genetic origins. There is strong epidemiologic and laboratory evidence for a causal association between new variant CJD and BSE.


A common, usually benign, herpesvirus that can cause life-threatening infection in immunosuppressed individuals.

Ecosystem disruption

Any perturbation to either the structure or function of an ecosystem.

Ectopic gene expression

Expression of a (trans)gene in a tissue where, or developmental stage when, such expression is not expected.


Introduction of DNA into a cell mediated by a brief pulse of electricity.

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