We developed a cell system where expression of thymidylate synthase (TS), an enzyme essential for DNA synthesis, can be modulated by a Zn2+-inducible promoter in MCF-7 cells, We found that overexpression of TS resulted in downregulation of p21 protein and mRNA levels. Statistical analysis demonstrated a significant downregulation of p21, but not a statistically significant decrease in p53 protein levels following TS induction. Since p21 is known to be transcriptionally activated by p53, these results suggest that TS downregulation of p21 may be occurring through a p53 independent mechanism in this in vitro cell system. In addition, cell cycle analysis demonstrated that down regulation of p21 by TS resulted in a decreased G(1)/S ratio in MCF-7 cells.