The impact of S-phase DNA on pulsed-field electrophoretic DSB measurements. Nocodazole-synchronised chicken DT40 pre-B cells were analysed 0–18 hours after removal of the drug. (A) Flow cytometric estimates of the S phase fraction. (B) Pulsed-field gel images of DNA migration following 40 and 10 Gy X-irradiation without repair incubation. (C): Fraction of DNA released (FAR) as a function of time after nocodazole removal. FAR values are inversely correlated to the fraction of S phase cells shown in the top diagram.