Toxicity in relation to residual γH2AX in SiHa cells exposed to MNNG. Panel a: Clonogenic surviving fraction after a 30 min exposure of SiHa cells to MNNG followed by a 24 hours recovery period. Panel b: Fraction of cells without γH2AX foci 24 hours after exposure to MNNG. Results are expressed relative to the endogenous expression of γH2AX; typically 60-70% of SiHa cells lack foci. Panel c: Comparison between fraction of cells lacking residual γH2AX foci and clonogenic fraction. Panels d-f: Distribution of γH2AX foci per cell. Panels g-i: Representative antibody-stained images of cells exposed 0, 0.5 and 1 μg/ml MNNG showing foci numbers and distribution.