On this page you can find a list of skills with a mutagen slot in The Witcher 2: Assassins of Kings. 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.It is possible to unlock all 13 Mutagen slot skills + Impregnation level 2 at exactly level 35. However, such a build will only be worthwile when using very powerful mutagens like Madness mutagen, Greater power mutagen or Greater critical effects mutagen. If you intend to use greater mutagens it's best to also unlock Side effects level 1 while working your way toward Impregnation. Even though you will only have 2% chance to gain mutagens through Alchemy it is still doable with patience. It's probably best to brew Insectoid Oil as you can brew it from Cortinarius and Celandine which are very common ingredients starting from Chapter I. Just brew around 30 one by one by spamming the "Enter" key and then reload the game if you don't get any, eventually you should get some.