Mewgenics Mutations Tier Guide: The 6 S-Tier Picks Every Run Wants

Mutations are how a good cat becomes an elite cat. The breeding system gives you stats and abilities, but the run-defining swing comes from a short list the community has tested into S-tier. Six mutations carry the meta. Most of the other 200 are filler, and a handful actively brick a run.

This guide ranks the heritable mutations from the 208-entry catalog and shows you how to chase the picks you want through breeding, furniture, and the Mutation room stat.

Three Mutation Categories

The catalog splits into three buckets, and the split tells you where the value comes from.

Ability mutations (96 entries). Add a mechanic to your cat. Pikachu Tail replaces movement with a teleport. Pointy Ears converts every basic attack into a 1 HP heal. A single inherited ability can change how a class plays, which is why this category dominates S-tier.

Stat mutations (71 entries). Shift raw stat values. Bear Leg adds +1 STR / +1 CON. item:head-long-neck adds +1 Range. The +2/-1 stat-trade family covers most of this category (9 body slots times 42 IDs gives 378 variants); the catalog reduces them to 3 representative entries because the choice is just "which stat do you want."

Behavior mutations (41 entries). Change how the cat acts in or out of combat. Some are upside, like item:mouth-gills granting +2 Health Regen while wet. Many are pure downside, like Red Eyes accumulating 5% miss chance per turn. This category is where most run-killing rolls live.

S-Tier: The Six Defining Mutations

Every published tier list and every meta breeding guide flags these six.

Pikachu Tail and Sonichu Tail

Movement becomes shadowstep. The cat ignores walls, terrain, and enemy zones of control, and teleports up to its Speed value in any direction.

The single most powerful repositioning tool in the game. Turns a item:hunter glass cannon into a flanker that cannot be locked down, and lets a item:cleric reach a downed ally across the map in one action. item:tail-sonichu-tail (tail.757) has identical mechanics. The catch: shadowstep only triggers on the standard Move action, so abilities with their own movement resolve normally.

Dice Eyes

End of every turn, the cat gains a random stat buff. No cap, no downside, no trigger beyond surviving the turn.

Buffs roll across the seven base stats, so results are unpredictable. Worst case is free stacking power. Best case is a 7 SPD cat sitting at 12 SPD by turn five. The wiki documents this as unlimited scaling, and SlashSkill flags it as one of the few mutations with zero downside. Stack item:eye-dice-eyes with item:tail-exhaust-pipe and you get a guaranteed +1 SPD plus a coin-flip on a second stat every turn.

Exhaust Pipe

+1 SPD at the end of every turn, cumulative across the battle. Same trigger as Dice Eyes, but the value is locked instead of random.

A 4 SPD item:tank with Exhaust Pipe ends turn three at 7 SPD and turn six at 10 SPD. That fixes the class's core problem of acting last and never repositioning to peel. The mutation lives in the tail slot, so it competes with Pikachu Tail. Mobility classes want Pikachu. Frontline classes want Exhaust.

Pointy Ears

Every basic attack heals the cat for 1 HP. Passive sustain with no setup.

The 1 HP looks small until you stack it against a item:hunter or item:butcher dishing 3-4 basic attacks per round. The heal doubles for item:monks because the class triggers extra basic attacks through its punching mechanic. A bred Monk with Pointy Ears is the closest the game has to a self-healing duelist. The mutation does not trigger on ability damage; a item:mage casting Magic Missile gets nothing.

Rat Mouth

Basic attacks apply +1 Poison. Free poison on every swing, no chance roll, no cooldown.

Poison bypasses armor and stacks with itself, so a item:mouth-rat-mouth item:hunter lays 4 Poison across a four-attack turn and the target loses 4 HP at end-of-turn before any other status ticks. Damage compounds against bosses with deep HP pools and any encounter past three rounds. Pair with item:mouth-shark-mouth (A-tier, +1 Bleed on basic) and the layered DoTs do more damage than the basic hits themselves.

Bear Leg

+1 STR and +1 CON. No downside.

The Steam meta breeding guide's recommended first mutation to breed: zero penalty, two combat-relevant stats. Most stat mutations are +2/-1 trades. Bear Leg is +2/0, which is what puts it at S instead of B. A 5/5 STR/CON kitten becomes a 6/6 cat with no other breeding work.

A-Tier Picks Worth Chasing

These mutations did not crack S because they carry a downside or a narrower use case. Meta breeding guides flag every one as "take it if you see it."

Exposed Brain. +1 permanent INT post-battle, plus +2 Bruise at battle start. The bruise is a 4 HP cost up front, but permanent INT scaling is the only mutation in the catalog that grows the cat between fights. item:cleric and item:psychic archetypes cite item:head-exposed-brain as a primary target. Overrides ear mutations with a -2 DEX penalty, so keep it off item:hunters and item:thiefs.

Long Neck. +1 Range. Stacks with item:eyebrow-extra-eye for +2 total. The cleanest non-class way to extend a item:mage's threat circle by a tile.

Mr. Worm. Allows unequipping cursed items and parasites. The Steam meta breeding guide flags item:tail-mr-worm because cursed item lockup is one of the few hard fail states. A safety pick that pays for itself the first time a cursed item rolls onto your carry.

Hippo Teeth. Heal 1 HP on any damage you deal. Triggers on ability damage, unlike Pointy Ears (basic only). item:mouth-hippo-teeth wins on ability-damage classes (Mage, Psychic, item:necromancer). Pointy Ears wins on basic-attack classes (Hunter, item:butcher, item:monk).

How to Chase a Specific Mutation

Three mechanics control whether a kitten gets the mutation you want.

Inheritance from parents. Mutations carry through breeding. A parent with Bear Leg has a chance to pass it. Both parents carrying the same mutation roughly doubles the odds. Mutation inheritance follows the same parent-roll structure as ability inheritance, so Stimulation biases the result toward the better parent. For the full breakdown of how the parent-roll math works, see our Breeding Guide.

Mutation room stat. The Mutation house stat governs overnight mutation events on cats housed in the room. The wiki documents it qualitatively: high Mutation produces more frequent random rolls, low Mutation keeps outcomes predictable. The single highest documented Mutation furniture piece is Brain of Glorg at +4 Mutation, which costs you 4 Appeal house-wide. Stack it with Black Box, Boiling Flask, and Ooze Canister to push a dedicated mutation room into the +8 to +10 range.

Stimulation level. Stimulation controls the better-parent bias during inheritance. At Stim 50, better-parent probability is 60.0%; at Stim 100, 66.7%. If only one parent carries the target mutation, raising Stimulation pushes the odds toward that parent. If both parents carry it, the Stimulation lever stops mattering for that specific trait.

The wiki does not publish the exact per-night Mutation roll formula, so treat the stat as a "more is better" knob rather than a precise probability dial.

Mutations to Avoid

The bottom of the catalog is where runs go to die.

Red Eyes (Cataracts). +5% miss chance per turn, accumulating. By turn 10 the cat misses one in two attacks. No removal short of breeding it out.

Floppy Ears (Wilted Ears). Battle starts with Immobile 1. Turn one lost. Run-ending on a item:tank against any boss with an opening burst.

Inward Eyes / Low Ears. Confusion 2 at battle start. The cat targets random units, including allies, until the status clears.

No Mouth. Disables consumables, food pickups, and musical abilities. item:mouth-bandage-mouth is the same effect in cosmetic form. Both block the item:butcher's food-drop sustain and lock out catnip recovery.

Spider Eye versus Spider Eyes. Two distinct mutations in the data despite near-identical names. item:eye-spider-eye (eyes.1029) is +5% crit chance. item:eye-spider-eyes (eyes.308) is +5% Dodge. SlashSkill's tier list conflates them as a single "+5% crit" entry, which contradicts the wiki. For crit, target Spider Eye. For dodge, target Spider Eyes.

Breeding Strategy Implication

The meta playbook runs in three phases. Generations 1-2 lock in stat floors: no stat below 5, no behavior defects. Generation 3 brings in the first chase mutation, with Bear Leg as the standard pick. Generations 4-6 chase the class-specific S-tier ability: Pikachu Tail for mobility, Pointy Ears or item:mouth-rat-mouth for damage, item:tail-exhaust-pipe for frontline.

Past generation 6, the puzzle stops being about adding mutations and starts being about not losing them. Each new pair risks a behavior defect that overwrites a slot you already locked in. The high-Mutation room that helped you fish for Bear Leg in generation 3 starts costing you Pikachu Tail in generation 7. Drop the cat into a low-Mutation room once the lineage has its targets.

This is the late-game theorycrafting puzzle the breeding system is built around. The best cat in your roster is one mutation roll away from being one roll worse. The math overlaps heavily with raw stat rolls; for the cleanest treatment of how STR, DEX, and the rest carry across generations, see our Stat Inheritance Guide.

What's Next

The next research pass needs the exact per-night Mutation probability curve and the class-specific ability/mutation pairings. Until that data lands, the six S-tier picks above carry the run. For how each class actually uses these mutations in practice, see our Class Tier List.