This chapter describes a method to isolate γ-ray hypersensitive mutants of Arabidopsis thaliana from ethyl methanesulfonate- (EMS) treated seed ( 1 ). The mutants are identified by visible symptoms of extreme radiation damage 10–15 d following exposure of seedlings to a threshold 10-krad dose (Fig. 1 ). Absence of meristem growth is the most visible symptom of extreme damage. Thus, it is used as the primary selection criterion. The method, therefore, targets single-gene EMS-induced mutations important for restoration of growth in radiation-damaged plants.   Fig. 1.  Test system for evaluation of radiation damage in Arabidopsis ( A ). Appearance of 3- to 5-d-old seedlings at the time of γ-ray exposure ( B ). Growth of 15-d-old nonirradiated Arabidopsis plants for comparison ( C ). Growth inhibition of 15-d-old rad mutant (arrow) and wild-type plants by γ-rays (10 krads). Note difference in expansion of the cotyledons of the γ-ray hypersensitive mutant and “weak” plants that fail to grow for other reasons.