Toxicity, DNA damage, and γH2AX formation in V79 cells exposed to MNNG. Panel a: Clonogenic survival after a 30 min exposure of V79 cells to MNNG. Panel b: Expression of γH2AX in V79 cells relative to untreated cells as a function of dose of MNNG. Panel c: Time-dependence of γH2AX formation in V79 cells after exposure to low doses of MNNG. Panel d: Alkali-labile lesions measured using the alkaline comet assay showing drug dose dependence for different times of recovery after a 30 min exposure. Panel e: DNA double-strand breaks measured using the neutral comet assay as a function of drug dose and time after a 30 min exposure. Panel f: Rejoining of alkali-labile lesions as a function of time after exposure of V79 cells to MNNG.