The following abilities have mutagen slots:
The last 2 in the Witcher training tree
The last 3 in the Swordsman tree
The last 5 in the Alchemy tree
The last 2 AND the Enhanced Yrden in the Magic tree
Once you have used a mutagen, it will remain there forever. You can NOT remove or upgrade it.
The mutagens have these effects (lesser/normal/greater):
Vitality: +2 vitality/+5 vitality/+10 vitality
Strength +1 armor/+2 armor/+2 armor and +1 vitality regeneration (outside combat)
Enhancement: +1 adrenaline/+3 adrenaline/+5 adrenaline on hit
Critical effects: +1%/+2%/+3% chance of all critical effects
Power: +1 damage/+1-2 damage/+1-3 damage
Concentration: +1 sign damage
Range +2%/+5% Aard and Igni range. This is a percentage increase and so will usually only increase the range of an already upgraded sign by 1.
Madness: +2 armor and +5 vitality and +1-2 damage. The damage bonus is not listed until you perform the mutation.
I have not seen lesser or greater Concentration mutagens. Since a normal one gives +1 sign damage and it is hard to see how it could be less, I doubt they exist.
I have not seen a greater Range mutagen. If it exists, it probably has +10% range.
I have not seen lesser or greater Madness mutagens. If they exist, they probably have the effects of Strength+Vitality+Power mutagens of the same strength.
The Impregnation talent in the Alchemy tree says it gives +15%/+35% effects of mutagens. It does a lot more than that. Level 2 is roughly a doubling, but it varies by type. Note that mutagens you have already used do NOT get boosted. If you want to get this talent, do it before you use any mutagens.
These are the effects of GREATER mutagens with 1/2 levels in Impregnation:
Vitality: +15 health/+20 health
Strength +3 armor and +1 vitality regen/+4 armor and +1 vitality regen
Enhancement: +7% adrenaline/+10% adrenaline
Critical effects: +4%/+6% critical effects chance
Power: +2-4 damage/+3-5 damage
Concentration (normal): +2 sign damage/+3 sign damage
Range (normal): +7%/+9% Aard and Igni range
I did not have a Madness mutagen available for this testing.