Incorporation of [³H]ara-C, [³H]dFdC, and [³H]F-ara-A into DNA. Approximately 1.5 × 10⁷ exponentially growing CCRF-CEM cells suspended in 50 mL culture medium were labeled with: (A) [³H]dThd (0.2 μCi/mL), (B) [5-³H] dCyd (0.2 μCi/mL), (C) 0.6 nmol/L [³H]ara-C (1.5 × 10⁻² μCi/mL), (D) 0.8 nmol/L [³H]dFdC (1.5 × 10⁻² μCi/mL), and (E) 120 nmol/L [³H]F-ara-A (1.5 μCi/mL) for 13 hours in the presence of 1.0 μmol/L BrdUrd and 10 nmol/L FdUrd. DNA isolation and CsCl ultracentrifugation were performed as described in Materials and Methods. Fractions of 0.2 mL were aspirated from the top of the gradients and diluted for the measurement of radioactivity (○) and UV absorption at 260 nm (×).