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X Chromosome

Type III hyperlipoproteinemia and some forms of apoA–I deficiency. ▶HDL-Dyslipidemia

Definition Women carry two X while men carry an X and a Y chromosome. X and Y collectively are called sex chromosomes. ▶Fragile X Syndrome ▶Repeat Expansion Deseases

Xanthophore Definition

XALD

Xanthophore is a neural crest-derived cell that is pigmented yellow. ▶Mutagenesis Approaches in Medaka

▶X-Linked Adrenoleukodystrophy

X-Chromosome Inactivation Xanthelasmata Definition Xanthelasmata are tiny (1–2 mm) yellowish plaques that are slightly raised on the skin’s surface of the upper or lower eyelids. They are a distinguished form of xanthomas by their specific localization. ▶Familial Hypercholesterolemia

B RYAN K. S UN , J EANNIE T. L EE Howard Hughes Medical Institute, Department of Molecular Biology, Massachusetts General Hospital, Department of Genetics, Harvard Medical School, Boston, MA, USA [email protected] [email protected]

Synonyms Lyonization; X-chromosome silencing; Sex chromosome silencing

Xanthoma Definition Xanthoma originates from a lesion of foam cell (cholesterol) accumulation in skin or tendon. Skin xanthomas in eyelids are called xanthelasma. Xanthoma is seen in familial hypercholesterolemia, cerebrotendinous xanthomatosis sitosterolemia (phytosterolemia),

Definitions X-chromosome inactivation (XCI) is the process by which one of two X-chromosomes is transcriptionally silenced in female mammals, equalizing the dose of X-linked genes in females (XX) and males (XY). XCI takes place early in embryonic development when each cell chooses to inactivate one of the X-chromosomes. After the choice has been made between the two, all daughter cells maintain the same inactivated X (Xi). As a result, all females are ▶mosaics, composed of two

2014

X-Chromosome Inactivation

populations of cells that express the genes from one or the other X-chromosome. The process of X-chromosome inactivation is unique to mammals, but is only one of several known mechanisms of ▶dosage compensation found in organisms such as the mouse, fruit fly and nematode worm. One intriguing characteristic that is common to dosage compensation in mammals and fruit flies is the essential role of ▶noncoding RNAs in the process. From a medical perspective, the mechanism and pattern of XCI has important effects on the phenotype of X-chromosome ▶aneuploidies and X-linked genetic diseases. Emerging evidence suggests that XCI also has implications in cancer and other areas of medicine as well.

Characteristics Forms of X-Chromosome Inactivation Two forms of X-chromosome inactivation have been characterized in mammals, imprinted X-inactivation and random X-inactivation (Fig. 1). From an evolutionary standpoint, the ancestral form was probably imprinted X-inactivation, in which the father’s Xchromosome is always silenced and the mother’s X-chromosome remains active. To distinguish the two

X-Chromosome Inactivation. Figure 1 Imprinted and random X-inactivation. After early cleavage stages, cells of the zygote separate int