“…The use of this system makes it possible to collect a large number of samples relatively easily and to detect all pink mutant cells occurred without being concealed by other cells (Ichikawa, 1992). Because of these merits, this system has been successfully used for studying the variation of spontaneous mutation frequency (Sparrow and Sparrow, 1976;Takahashi and Ichikawa, 1976;Mericle et al, 1976;Nauman et al, 1978;Ichikawa et al, 1981Ichikawa et al, , 1996aIchikawa et al, , 1996cIchikawa, 1984;Imai et al, 1991;, as well as for detecting mutagenic synergisms among several chemical mutagens and X rays (CebulskaWasilewska et al, 1981;Ichikawa, 1992;Ichikawa et al, 1993;Shima and Ichikawa, 1994, 1997Xiao and Ichikawa, 1995).…”