DFdC and dFdU levels in plasma will be measured using reverse phase HPLC, as described by Grunewald (22). Briefly, 20 microl to 50 microl of plasma is injected onto a C18 mBondapak analytical column (Waters Associates, Inc.). Components will be separated with a mobile phase consisting of a 30 minute linear gradient starting with 100% solution A (0.5M ammonium acetate, pH = 6.8) and ending with 60% solution B (50% methanol in deionised water). Flow rate is at 1.5 ml/min. Detection wavelengths are 275 nm and 262 nm for dFdC and dFdU, respectively.