A comparison of mutation induction at the tk and hprt loci in human lymphoblastoid cells; quantitative differences are due to an additional class of mutations at the autosomal tk locus

X-Rays, ethyl methanesulfonate and ICR-191 induced 2 classes of trifluorothymidine-resistant mutants at the autosomal tk locus in human lymphoblastoid cells. These classes were differentiated by their growth rates; some mutants grew with a normal doubling time of 14-18 h (tk-NG), while others grew much more slowly, with doubling times of 21-44 h (tk-SG). Only mutants with normal growth rates were observed at the X-linked hprt locus; the frequencies of mutations induced at hprt were equal to those induced for tk-NG mutants. Thus, more mutations overall (by up to a factor of 6) were induced at tk than at hprt. These results are discussed in relation to recent studies in rodent cells, in which much greater mutation frequencies were observed at autosomal loci.