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.
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.