Mitotic index (MI), defined as the ratio (%) of the mitotic-phase-arrested cell number to the total cell number, is higher in the presence of
colcemid than in its absence. MI is reduced by anti-
tumor agents used in conjunction with
colcemid. The ratio of MI, which is defined by the dose of
colcemid to that of the supplement with
etoposide as an anti-
tumor agent and is designated MIR, was 10% at an exposure of 2 h, suggesting that
etoposide blocks the cell cycle before 2 h of the mitotic phase. Thus, we established a convenient method to identify a target point in the cell cycle for anti-
tumor agents using
colcemid. RBL-2H3, a mammalian cell line, exhibited a large number of 2-fold swollen nuclear envelopes after exposure to
etoposide. The G2-index, which is the ratio of the number of swollen nuclei to the total number of nuclei, was higher when the cells were incubated with
etoposide than without it. In this study, we suggest that the measurement of the G2-index contributes to the screening of putative anti-
tumor agents.