In summary, our experimental data indicate that, constitutive TYMS mRNA transcription and cell cycle-induced hTS regulation (i.e. increasing the amount of cells in the S phase) together with hTS enzyme stability [20], are the three key mechanisms that mediate 5-fluorouracil induced up-regulation of human thymidylate synthase expression in the two ovarian cancer cell lines studied. The gene discussed is TYMS; the disease is ovarian cancer.