x X̄ (X bar) X chromosome x prime x-axis x-coordinate x-intercept X-linked X-ray x′ Xanax xanthine xantho- (xanth-) xanthocyanopsia xanthomatosis xanthopsia xanthopsin Xenazine xeno- (xen-) xenoglossophilia xenoglossy xenophobia xenorexia xeroderma pigmentosum xerophthalmia XX XXX syndrome XXXX syndrome XXXXX syndrome XXXXY syndrome XXXY syndrome XXY syndrome XXYY syndrome XY xylene Xyrem XYY syndrome XYZ grouping XYZ system