A simple autoradiographic method for checking HGPRT-deficiency in colonies of mammalian cells

HGPRT enzyme activity in mutant colonies selected in 6-thioguanine can be assessed directly in petri dishes using an autoradiographic method. The application of this method to large-scale quantitative mutation experiments verifies that: (a) the maximum induced frequency of mutation of HGPRT-deficiency can be measured at short expression times, such as 3 days after treatment, when cells are respread into relatively low thioguanine concentrations, and (b) ionizing radiation induces predominantly mutants with zero HGPRT activity. Other potential applications of this method are discussed.