| Term | Definition / Meaning / Stand for |
| XALD |
X-Linked Adrenoleukodystrophy |
| XBP |
X-box Binding Protein |
| XCM |
X-ray Crystallographic Model |
| XCS |
X-ray Crystallographic Structure |
| XCS |
X-ray Crystallography System |
| XRCE |
X-ray Enhanced Crosslinking Enzyme |
| XRFS |
X-ray Fluorescence Spectroscopy |
| XIS |
X-ray Induced Synthesis |
| XIC |
X-ray Ionization Chamber |
| XPOL |
X-ray Polymerase |
| XRCC1 |
X-ray Repair Cross-Complementing Protein 1 |
| XRCC2 |
X-ray Repair Cross-Complementing Protein 2 |
| XRCC3 |
X-ray Repair Cross-Complementing Protein 3 |
| XRCC4 |
X-ray Repair Cross-Complementing Protein 4 |
| XRCC5 |
X-ray Repair Cross-Complementing Protein 5 |
| XRCC6 |
X-ray Repair Cross-Complementing Protein 6 |
| XRCC |
X-ray Repair Cross-Complementing |
| XSC |
X-ray Structure Calculation |
| XSCA |
X-ray Structure of Carbohydrate-Active Enzymes |
| XUV |
X-ray Ultraviolet |
| XC |
Xanthine Crystals |
| Xpt |
Xanthine Phosphoribosyltransferase |
| XGPRT |
Xanthine-Guanine Phosphoribosyltransferase |
| XMP |
Xanthosine Monophosphate |
| XA |
Xanthurenic Acid |
| Xb |
Xenon Bond |
| XZ |
Xenon-Zeolite |
| XENP |
Xenotropic Envelope Protein |
| XTA |
Xenotropic Tumor Antigen |
| XPA |
Xeroderma Pigmentosum Group A |
| XPB |
Xeroderma Pigmentosum Group B |
| XPC |
Xeroderma Pigmentosum Group C |
| XPD |
Xeroderma Pigmentosum Group D |
| XPF |
Xeroderma Pigmentosum Group F |
| XPG |
Xeroderma Pigmentosum Group G |
| XOC |
Xylan Oligosaccharide Chromatography |
| XPL |
Xylan Phosphorylase |
| XRBP |
Xylan Regulatory Binding Protein |
| XSE |
Xylan Specific Endoglucanase |
| XUS |
Xylan Uptake System |
| XulA |
Xylan Utilization Locus A |
| XSDE |
Xylan-Specific Decarboxylase Enzyme |
| XYNB |
Xylanase B |
| XynC |
Xylanase C |
| XYC |
Xylanase C |
| XCA |
Xylanase Carbohydrate-Active Enzyme |
| XSER |
Xylanase Esterase |
| XSE |
Xylanase Substrate Enzyme |
| XYS |
Xylanase System |
| XYN |
Xylanase |
| XT |
Xylanotransferase |
| XAD |
Xylene Adsorbent Resin |
| XplA |
Xylene Monooxygenase |
| XYCOS |
Xylene-Coordinated Organometallic System |
| XLB |
Xylene-Lysin Bromide |
| XLDH |
Xylitol Dehydrogenase |
| XGA |
Xyloglucan Arabinogalactan |
| XED |
Xyloglucan Endo-Diaminase |
| XIP |
Xyloglucan Inhibitor Protein |
| XYSA |
Xyloglucan-Specific Alpha-Amylase |
| XOS |
Xylooligosaccharide |
| XSH |
Xylose Hydrogenase |
| XylA |
Xylose Isomerase |
| XOX |
Xylose Oxidase |
| XLO |
Xylose Oxygenase |
| XYLP |
Xylose Permease |
| XyP |
Xylose Phosphate |
| XRE |
Xylose Reductase Enzyme |
| XS |
Xylose Synthase |
| Xup |
Xylose Uptake Pathway |