Background: Thymidine kinase 1 (TK1) catalyzes the phosphorylation of thymidine to maintain dTTP in the cell for DNA replication and repair. The salvage of thymidine by TK1 is cell-cycle regulated and important to genomic stability. TK1 is a phosphoprotein and exists in a dimeric or tetrameric form. Phosphorylation of TK1 may be important to its quaternary structure and the regulation of its catalytic activity.