The Salmonella typhimurium TA 98 (frameshift) and TA 100 (base-pair substitution) strains
which were histidine dependent due to a mutation of the histidine operon, were kindly provided
by the research division of the National Cancer
Institute, Bangkok, Thailand, and maintained as
described by Maron and Ames (1983). The genotypes of the test strains were checked routinely
for their histidine requirement.