On this page you can find a list of skills with a mutagen slot in The Witcher 2, there are only 13 of these. To mutate a skill with a mutagen, just press the 
key or enter the meditation menu and click on "Character". Next, choose the skill and press 
to see a list of available mutagens, then select the one you wish to apply and press 
.
You need at least one talent invested in a skill to mutate it.
Note: Once the skill has been mutated, you cannot remove or replace the mutagen.
See also:
