DNA substrate (1,740 bp) containing a single BamHI and SspI recognition sequence was generated by PCR. Five hundred nanograms of DNA were incubated with test substances or solvents for 16 h at 37 °C in 10 mM Tris⋅HCl, pH, 1 mM EDTA. Treated DNA was incubated with 20 U of endonuclease for 3 h at 37 °C, and then resolved electrophoretically.