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X CHROMOSOME INACTIVATION (Syn: lyonization) A process by which one of the two copies of the X chromosome present in female mammals is inactivated by packag- ing in repressive heterochromatin. X-inactivation occurs so that the female, with two X chromosomes, does not have twice as many X chromosome gene products as the male, which only has a single copy of the X chromosome. See also epigenetic inheri- tance.

XDR (EXTENSIVELY DRUG-RESISTANT TUBERCULOSIS) See drug-resistant tuberculosis, extensively.


  1. A chemical compound that is foreign to a biological or an ecological system. A substance, typically a synthetic chemical, that is foreign to the body. Examples include many synthetic pesticides and their derivatives, food additives, or persistent toxic substances such as dioxins and polychlorinated biphenyls (PCBs).219 See also carcinogen.
  2. (Syn: commensal, symbiosis) Pertaining to an association of two animal species, usually insects, in the absence of a dependency relationship, as opposed to parasitism. Contrast endobiotic. See also commensal.

XENODIAGNOSIS Detection of a (human) pathogenic organism by allowing a nonin- fected vector (e.g., mosquito) to consume infected material and then examining this vector for evidence of the pathogen.

X-LINKED (Syn: sex-linked) Heritable characteristic transmitted by a gene located on the X chromosome.

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