TK1 (thymidine kinase 1) is a cell-cycle-regulated enzyme catalyzing the first step of thymidine salvage, converting thymidine to thymidine monophosphate 1. Expression is transcriptionally limited to S phase, coinciding with dTTP oscillations. Beyond nucleotide metabolism, TK1 plays critical roles in telomere maintenance—TK1 deletion decreases telomere length, while thymidine supplementation drives telomerase-mediated telomere elongation 2. TK1 also activates anticancer and antiviral nucleoside analogs including AraC and AZT 1. In cancer biology, TK1 is significantly overexpressed across multiple malignancies and serves as a prognostic biomarker 3. TK1 promotes tumor progression through multiple mechanisms: it enhances cellular proliferation by regulating G0/G1 phase arrest 4, mediates replication stress in pancreatic cancer via dTTP accumulation 5, and coordinates tumor immune evasion by modulating Th2 cell polarization via CCL5 chemokine signaling 4. In lung adenocarcinoma and triple-negative breast cancer, TK1 knockdown impairs proliferation, migration, and invasion while restoring immune function 67. TK1 mRNA stability is enhanced through m5C methylation-dependent mechanisms 8. Clinically, TK1 represents a promising CAR T-cell immunotherapy target and biomarker for predicting immunotherapy response and overall survival across cancer types.