Abacavir, a nucleoside analogue reverse transcriptase inhibitor, has been found to exhibit an anti-cancer activity on ATL cells in vitro without inhibiting DNA replication of normal cells. Abacavir or a pharmaceutically acceptable derivative thereof is useful as an active ingredient of a pharmaceutical composition for use in the prevention or the treatment of cancer, in particular a cancer whose DNA repair system is impaired such as breast cancer or adult T-cell leukemia.
