I don't know if radiation causes different types of mutation to other
things, I may well do. But I know that the researchers who are studing this
would know whats best and they would use that method. They are interested
in this topic, and would do all they can to get the results they want.
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And these results are ? As far as I can tell they are interested in how the different genes are expressed. Nothing sinister about that. Radiation is a very good method for studying this.