XAF1 (XIAP-interacting protein 1): a protein that can antagonize the anticaspase activity of XIAP and is regulated in expression by interferons. Cancer cells express less XAF1 than normal cells, suggesting that this proapoptotic protein may have tumor suppressor function.
xenobiotic: a chemical found in the body that is not normally produced or expected to be there; substances foreign to a biologic system.
xenograft: host graft from a species that is not related to the recipient.
XIAP: see IAP (inhibitors of apoptosis proteins).
XRCC1 (X-ray repair complementing defective repair in): gene encoding the protein that is
involved in the efficient repair of DNA single-strand breaks formed by exposure to ionizing radiation and alkylating agents.