Thymidine Kinase also known as TK1 is a cytosolic enzyme that catalyzes the addition of a gamma-phosphate group to thymidine. TK1 plays a crucial role in DNA synthesis by catalyzing the phosphorylation of deoxythymidine to deoxythymidine 5′-phosphate, a key step in the nucleotide salvage pathway. This function is particularly important as TK1 facilitates the incorporation of deoxythymidine into DNA, which is essential for cell proliferation and repair. TK1 activity is significantly elevated in actively dividing cells, making TK1 a valuable biomarker for assessing cell proliferation in various clinical contexts, especially in the diagnosis and monitoring of hematological malignancies. Additionally, TK1 is critical for the efficacy of several antiviral drugs, such as azidothymidine (AZT), and is utilized in the selection of hybridoma cell lines during monoclonal antibody production.