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• Xanax
• A drug used to treat anxiety disorders and panic attacks. It is being studied in the treatment of nausea and vomiting caused by some cancer treatments. It is a type of benzodiazepine. Also called alprazolam.

• Xeloda
• A drug used to treat stage III colon cancer in patients who had surgery to remove the cancer. It is also used to treat metastatic breast cancer that has not improved after treatment with certain other anticancer drugs. Xeloda is being studied in the treatment of other types of cancer. It is taken up by cancer cells and breaks down into 5-fluorouracil, a substance that kills tumor cells. Xeloda is a type of antimetabolite. Also called capecitabine.

• xenograft
• The transplant of an organ, tissue, or cells to an individual of another species.

• xeroderma pigmentosum
• A genetic condition marked by an extreme sensitivity to ultraviolet radiation, including sunlight. People with xeroderma pigmentosum are not able to repair skin damage from the sun and other sources of ultraviolet radiation, and have a very high risk of skin cancer.

• xerogram
• A picture of the body recorded on paper rather than on film. Also called xeroradiograph.

• xeroradiograph
• A picture of the body recorded on paper rather than on film. Also called xerogram.

• xeroradiography
• A type of x-ray in which a picture of the body is recorded on paper rather than on film.

• xerostomia
• Dry mouth. It occurs when the body is not able to make enough saliva.

• XK469R
• A substance that is being studied in the treatment of leukemia. It belongs to the family of drugs called topoisomerase inhibitors.

• XL880
• A substance being studied in the treatment of cancer. XL880 blocks enzymes involved in the growth and spread of tumor cells. It may also prevent the growth of new blood vessels that tumors need to grow. It is a type of tyrosine kinase inhibitor and a type of antiangiogenesis agent.

• X-linked recessive
• X-linked recessive inheritance refers to genetic conditions associated with mutations in genes on the X chromosome. A male carrying such a mutation will be affected, because he carries only one X chromosome. A female carrying a mutation in one gene, with a normal gene on the other X chromosome, will be unaffected.

• XR9576
• A substance that is being studied in the treatment of cancer. It may help tumor cells respond again to drugs they have become resistant (unable to respond) to. XR9576 is a type of multidrug resistance inhibitor and a type of P-glycoprotein antagonist. Also called tariquidar.

• x-ray therapy
• A type of high-energy radiation. In low doses, x-rays are used to diagnose diseases by making pictures of the inside of the body. In high doses, x-rays are used to treat cancer.

• XRP9881
• A substance that is being studied in the treatment of breast cancer. It belongs to the family of drugs called taxane derivatives.

• Xyotax
• A form of the anticancer drug paclitaxel combined with a protein called poliglumex that may have fewer side effects and work better than paclitaxel. It is being studied in the treatment of breast cancer, ovarian cancer, lung cancer, and other types of cancer. It belongs to the family of drugs called mitotic inhibitors. Also called CT-2103, paclitaxel poliglumex, and paclitaxel polyglutamate.