Fig. 1. (a) Structures of Ru1 and dppz. Ru1 was used as the chloride salt and as a mixture of Λ and Δ enantiomers. (b) Representative images of DNA replication fibres in OE21 oesophageal cancer cells. CldU (first pulse, 30 min) incorporation is shown in red, incorporated IdU (second pulse, 30 min) in green. The second nucleotide (IdU) was either mock-treated or incubated in the presence of Ru1 or dppz to determine impact on DNA replication fork progression. Scale bar = 10 μm. (c) Quantification of IdU-labelled tract length in the presence of Ru1 (21 μM) or dppz (7 μM). Tract length was determined for >100 forks per condition. Whisker box plots show mean values and data within the 10–90 percentile. (d) Quantification of completely stalled forks (i.e. CldU tract only) for Ru1 or dppz treatment as in (b). Fork stalling was quantified for >300 forks per condition and experiment. The experiment was repeated three times. Data were analysed using unpaired two-tailed t test (*P < 0.1, **P < 0.01, ***P < 0.005).