inherent or acquired limitation of an organism’s ability to respond to the challenge of exposure to a specific xenobiotic substance.

Checkpoint pathway

Type of pathway used in intracellular signaling. Checkpoint pathways are induced in response to a cell’s own internal imbalance or errors in its synthetic activities. Induction of checkpoint pathways leads to a delay in certain synthetic processes until other processes are complete, thereby averting damage.


An animal consisting of genetically different cells derived from two (or more) different zygotes.


Structures that contain an organism’s genes.


The first few divisions of an embryo following fertilization. There is little or no growth during these divisions and the cytoplasm is cleaved into smaller and smaller units.

Complementary DNA (cDNA)

DNA copy of mRNA from expressed genes made by the enzyme reverse transcriptase.


Bacterial recombinase system in which the Cre protein mediates DNA recombination between specific DNA sequences known as lox-P sites. This system is used in mammalian cells to delete (or invert) a stretch of DNA by flanking it with lox-P sites and then exposing the cell to Cre protein at some predetermined time.

Deoxyribonucleic acid (DNA)

A complex macromolecule that is composed of nucleic acids (adenine, guanine, cytosine, and thymine) and is found in cellular organisms. DNA carries all the genetic information necessary to determine the specific properties of an organism. In its native state, DNA exists as a double helix.

Developmental defect

A structural or functional anomaly that results from an alteration in normal development.

Developmental toxicant

A physical, chemical, or biological agent that is shown to affect development under specific conditions of exposure.

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement