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"During the past seventy years, mutation breeding led to more than 2250 plant varieties (Maluszynski et al. [4]; Ahloowalia et al. [5]). 70% of these varieties were released as directly induced mutants, and the other 30% from crosses with induced mutants. The use of chemical treatments was relatively infrequent, but gamma rays were frequently used (64%), followed by X-rays (22%) (Ahloowalia et al. [5])." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2218926/