Deoxycytidine kinase (dCK) phosphorylates several anti-cancer and anti-viral nucleoside analogs. The enzyme is predominantly expressed in lymphoid tissues regulated by an unknown mechanism. We have cloned and sequenced the 20 kbp mouse dCK gene and approximately 1.7 kbp of the 5' flanking regions of both the human and mouse dCK genes. Five major inter-species conserved motifs were identified in the 5' region including the transcription initiation region, an SP1 site and two closely located putative octamer transcription factor sites. Luciferase reporter experiments showed that the human dCK 5' region efficiently initiated transcription but no tissue regulatory element could be identified.