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X chromosome
A sex chromosome; normal females carry two X chromosomes.
Xerostomia
A dry mouth condition; can be caused by a number of things, including rheumatoid arthritis, diabetes, kidney failure, infection with HIV (the virus that causes AIDS), drugs used to treat depression, and radiation treatment for mouth or throat cancer.
X-inactivation
The repression of one of the two X-chromosomes in the somatic cells of females as a method of dosage compensation; at an early embryonic stage in the normal female, one of the two X-chromosomes undergoes inactivation, apparently at random, from this point on all descendent cells will have the same X-chromosome inactivated as the cell from which they arose, thus a female is a mosaic composed of two types of cells, one which expresses only the paternal X-chromosome, and another which expresses only the maternal X-chromosome.
X-rays
High-energy radiation used in low doses to diagnose disease or injury, and in high doses to treat cancer.
XYY syndrome
Genetic condition in males with extra Y chromosome (in 1 in 1000 male births). Symptoms: tall stature (over 6'), may including sterility, developmental delay, learning problems.
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