Scoring of MN was limited to binucleate (BL) lymphocytes only with preserved cytoplasm according to the criteria proposed by Fenech et al. . Identification of binucleate cells in cell groups required careful visual examination of the individual cell boundaries. The results are expressed as number of micronucleated cells per 1000 binucleate cells on the two different slides from the same culture. Proliferation kinetics data was calculated by considering the frequencies of mono-, di-, tri-, and tetranucleate cells per each treatment group. A nuclear division index (NDI) was calculated according to the formula proposed by Takeshita et al.  as follows: NDI = (1M1 + 2M2 + 3M3 + 4M4)/N, where M1 to M4 represent the number of cells with one to four nuclei and N is the total number of cells scored.