Pitstop 2 causes cell death and inhibits cell growth in dividing cells. A, Asynchronously growing and G2/M-synchronized HeLa cells were subject to an LDH assay following a 20 h treatment with increasing concentrations of pitstop 2, dynole 34-2 and MLN8237. The graph shows the amount of LDH in each experimental condition, as an indicator of cellular cytotoxicity, which was calculated as a percentage of total cell lysis. B, G2/M-synchronized HeLa cells were exposed to increasing concentrations of dynole 34-2, MLN8237 and pitstop 2 for 24 h. Total viable and non-viable cell number (mean ± S.D. from two independent experiments) were assessed by trypan blue. C, Cells were treated as described in B. Lysates were collected and immunoblotted for cleaved PARP. HeLa cells exposed to UV served as a positive control. Actin served as a loading control. D, Quantitative densitometric analysis of cleaved PARP levels shown in C. Graph (mean ± S.E.M. from five independent experiments) shows the relative amount of PARP in HeLa cells following treatment with the indicated conditions that have been normalised to background.