Mutation Research, 129 (1984) 269-281 269 Elsevier MTR 03944 The nature of mutants induced by ionising radiation in cultured hamster cells I. Isolation and initial characterisation of spontaneous, ionising radiation-induced, and ethyl methanesulphonate-induced mutants resistant to 6-thioguanine Robert Brown * and John Thacker ** MRC Badiobiology Unit, Harwell, Didcot, Oxon OX 11 ORD (Great Britain) (Received 16 May 1984) (Revision received 10 July 1984) (Accepted 20 July 1984) Summaff A large number of thioguanine (TG)-resistant mutants of V79-4 Chinese hamster cells was isolated from untreated cultures and from cultures exposed to T-rays, a particles or ethyl methanesulphonate (EMS). Selection conditions were chosen to optimise the survival of all types of TG-resistant mutant, and the isolation procedure ensured that each mutant originated independently of any other. Hypoxanthine-guanine phosphoribosyl transferase (HGPRT) enzyme activity was measured in cell-free extracts of each mutant, and compared with repeat measurements made on the parental V79-4 cells and on a series of non-mutant clones. Ionising radiation-induced mutants were found to be mostly (or perhaps entirely) of the 'zero HGPRT activity' type, but about 20% of EMS-induced mutants and 50% of spontaneously occurring mutants had significant HGPRT activity. However, none of the TG-resistant mutants were found to lack activity of another X-chromosome-linked enzyme, glucose-6-phosphate dehydrogenase. Few mutants were found with visible X-chromosome changes, but the incidence of hyperploidy was higher among sponta- neous mutants than in the parental line and the induced mutants. Isoelectric focussing of cell extracts from those mutants which retained some HGPRT activity revealed several with shifts in the isoelectric points for HGPRT enzyme activity. Characterization of induced mutants of micro- organisms has shown that small molecular changes ('point mutations') such as transitions, transver- sions, and frameshifting deletions/additions of * Present address: The Beatson Institute for Cancer Re- search, Garscube Estate, Switchback Road, Bearsden, Glasgow G61 1BD, Great Britain. ** Correspondence to: Dr. John Thacker, MRC Radiobiology Unit, Harwell, Didcot, Oxon OXll ORD, Great Britain. 0027-5107/84/$03.00~1984 Elsevier Science Publishers B.V. DNA bases are caused by ionising radiations (Hartman et al., 1971; Phillips et al., 1972; Prakash and Sherman, 1973; Mailing and de Serres, 1973; Conkling et al., 1976; Glickman et al., 1980). With microbial mutation systems allowing a variety of genetic changes to be detected, larger genetic changes, often involving more than one gene, were also found to be induced by X- or T-rays (e.g. Webber and de Serres, 1965; Conkling et al., 1976). However, the ability of ionising radiation to in-