Relative independent mutations induced by vehicle (A), dibromoethane (DBE) (B), BSO/dibromoethane (C), or O6-BzGua/dibromoethane (D) in liver of Big Blue® transgenic mice. O6-BzGua (80 mg/kg, ip) was administered 1 h prior to treatment with dibromoethane (30 mg/kg, ip) and BSO (8 mg/kg, ip) was administered 2 h prior to treatment with dibromoethane (30 mg/kg, ip). Transition mutations (■); transversion mutations (□); frameshifts and others (diagonal shading). The P values are for comparison of the GC to AT transversions between Parts A and B (P <0.001) and between Parts B and C (P <0.001).