Inhibition of maternal Top2a in pre-MBT cycling extracts delays cell cycle progression by lengthening the M-phase. A) Cycling extracts from activated Xenopus eggs were treated with 20 μM Top2a inhibitor ICRF-193 diluted in 1% DMSO. To monitor cell cycle progression samples were taken every 10 minutes and phospho-histone H3 expression relative to GAPDH expression plotted. Blue and red bars indicate M phase in DMSO control and ICRF-193 treated extracts, respectively. B) Western blot analysis of phospho-Histone H3 and GAPDH expression levels. C) The increase in M phase length in two different cycling extracts treated with ICRF-193. M phase lengthened on average by 40% (SD = 3.14).