X XC XCS Xylene Cold Solubles XCS xylene cold soluble XG Xanthan gum XLA X-Linked Agammaglobulinemia XLD Xylose Lysine Deoxycholate XLRP X-linked retinitis pigmentosa XNA xeno nucleic acid XNA xeno nucleic acids XP xeroderma pigmentosum XPS XML paper specification XRD X-Ray Diffraction XRD X-ray diffractometry XRF X-Ray Fluorescence XRPD X-Ray Powder Diffraction XRPD X-ray Powder Diffractometer XRPD X-ray powder diffraction XRR X-ray reflection XRR X-ray reflectometer XSD XML Schema Definition