DCK is frequently inactivated in acquired gemcitabine-resistant human cancer cells

► We established GEM-resistant cell lines from 6 human cancer cell lines. ► There were no abnormalities on influx and efflux of GEM. ► Inactivating mutations of deoxycytidine kinase (DCK) were found in 5 of 6 GEM-resistant cell lines. ► Introduction of DCK into restored GEM sensitivities and knockdown yielded GEM resistance. ► Inactivation of DCK plays a crucial role for acquisition of GEM resistance.