ok, i have just checked this link :
http://www.kf-wiki.com/wiki/List_of_whitelisted_mutators
the problem is that i would never have the idea, for example, to fight against fleshpounds only without at least a level 5, the "official mutators" are useless.
we should be able to select our perk, but it should block achievements and leveling.
I have made this post mainly to say that, but I had to be sure that it was not a bug (when I see the weird problem that I have
http://forums.tripwireinteractive.com/showthread.php?t=67093 ), with the hope that tripwire will take a look at it