AQP9 and cancer: Because 5-FU exerts its anticancer effect through incorporation of FdUTP into DNA and RNA specifically during S phase,21 cancer cells in the S phase are more susceptible to 5-FU-induced cell death.22 We therefore sought to determine whether AQP9 overexpression has an effect on cell cycle regulation and, more specifically, whether it increases the toxicity of 5-FU.