RequiredFieldValidator on MutuallyExclusiveCheckBoxExtender

Hello all,
 
I have several MutuallyExclusiveCheckBoxExtender's on different lists of check boxes. They work fine. Now I need to make sure a selection is made. How can I do this? Yes I know that a radiolist or listbox would be easier but I dont want to go that route. I need to do it with check boxes.
 
Thank you
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