Once you collect enough EXP, or "Rads", you will level up, and upon finishing the level, be able to choose 1 out of 4 different "perks" called mutations.

Each has unique effects, and unlike other games, most of them are situational, and do not guarantee a won run everytime you take them.

This guide exists to shed some more light.

Long Arms
More Melee Range

Its a weapon specific, so only pick it up if you have a melee weapon.
Its a great mutation, as it helps you get rid of the main issue of Melee weapons, their range.
(Also if you have this and you skip an energy screwdriver that drops, you dont deserve to have the energy screwdriver drop to you)

Arrow's Comments: This mutation is AMAZING on loops as you can hit 2x more enemys per swing 4/5


Boiling Veins
Immunity to fire and explosion damage when on 4 HP and below

TAKE IT. On some characters its more useful, but this mutation is amazing for everyone. This makes the scrapyard(the third area) easier, as the fire traps and the salamanders(who shoot fire) wont be able to kill you, and this mutation makes explosives, who are risk vs. reward, unable to kill you, meaning the risk is much smaller.

Late and late mid-game are also full of explosions that can 1-shot you(mostly bosses), and this mutation makes explosions stop being a problem.

Arrow's Comments: Not THAT good late game as there are not that many explosive enemys on loop but it can save you from rouge explo freaks 2/5

Bolt Marrow
Homing Bolts

Its a weapon specifics mutation that isnt a must, but if you have a super splinter gun, a splinter gun, an auto crossbow, super crossbow, or just about every bolt weapon(Except for Disc Gun, which can go burn itself in hell), this mutation can be quite fun and helpful.

You know how if enemies are in a straight line and you shoot both of them with 1 bolt? Bolt marrow makes the bolts pierce enemies and go zigzag sometimes to hit more enemies. Super Splinter Gun with Marrow = Godmade




Laser Brain
Energy Weapons Do More Damage

Its a weapon specific, so only pick it up if you have an energy weapon.
It doesnt just make them do more damage, it also makes their attacks last longer. It can be really helpful in some cases, and trivial in others. The wiki talks about it in more length.



Recycle Gland
Most Hit Bullets Become Ammo

Its a weapon specific for bullets(Except for Pop Weapons, so pop gun and Pup Rifile wont work with this) that has a 60% chance of giving you back a bullet when it hits something. If you use a Heavy Bullet weapon, it has a 60% to give you 2 bullets back when it hits something.

Makes weapons like the Double Minigun actually usable without running out of ammo in a few seconds.




Shotgun Shoulders
Shells Bounce Further
Weapon specific for Shells.
Makes shells(and the bullets that Pop Gun and Rifile shoot) bounce more.

Its not amazing, but it can do some crazy things sometimes.



Heavy Heart
More Weapon Drops
After picking 3 Weapon Specific mutations, Heavy Heart will pop up.
It will never come back, as it appears only once, in the next time you can choose mutations after taking the third weapon specific mutation.

Makes the chance of enemies dropping weapons upon death higher.

Hammerhead
Can Push Through Walls

Early game, this mutation pretty much means nothing. Mid-Game, it can save you a good amount of times. Late Game and Late Mid-Game....If you dont take it, you're gonna die a lot more easily than if you did.

Basically, it allows you to break 15 walls every level by pushing yourself into them, and thus make cover/ambush enemies. You can also use it to destroy the fire traps in The Scrapyards.


Euphoria
Slower Enemy Projectiles

It makes more bullets be on screen, but it also makes them slower. If you're Eyes and you have Throne Butt with this, you're practically in the Matrix.

I remember seeing a lot of hate on it, but its been some time without people acting like it's trash, and personally I take it and it helps quite a few times.


Rhino Skin
+4 HP

Gives you 4 more HP to your max HP. Can also be used to revive Chicken if she's headless in a portal.
Very useful for obvious reasons.


Strong Spirit
Prevents 1 Lethal Hit.
Recharges at Full Health in the Next Area

Say you're on 1 HP and you get shot. In that moment, if you have Strong Spirit, instead of dying, your halo will disappear.
When you get back to full HP and are in the next area/level(Both work), Strong Spirit comes back, and you can see that thanks to the halo.


Gamma Guts
Enemies Touching You Take Damage

I LOVE Gamma Guts. It used to be utter trash, then OP for 1 update, and then it became what it is now.

If an enemy that has no contact damage or lower hp than the damage Gamma Guts makes you do when you touch an enemy, upon death, it'll not hurt you at all and would be 1 shot.
When an enemy is killed by Gamma Guts, The green blast you see deals 8 damage to all nearby enemies.

Gamma Guts makes Labs and Sewers and Loop desert into a joke, as they include many enemies who die to it easily.
(Oh, and when snowbots from the frozen city aren't charging, they do no contact damage. Yes, GG can kill them like that)



Sharp Teeth
Being Hit Deals Damage to All Enemies On Screen
Just what the description says, only that now(Update 96) it seems to have a small delay before hitting enemies.

Honestly, I prefer Gamma Guts, but this in no way means that this mutation is bad.

Gamma Guts
Enemies Touching You Take Damage

I LOVE Gamma Guts. It used to be utter trash, then OP for 1 update, and then it became what it is now.

If an enemy that has no contact damage or lower hp than the damage Gamma Guts makes you do when you touch an enemy, upon death, it'll not hurt you at all and would be 1 shot.
When an enemy is killed by Gamma Guts, The green blast you see deals 8 damage to all nearby enemies.

Gamma Guts makes Labs and Sewers and Loop desert into a joke, as they include many enemies who die to it easily.
(Oh, and when snowbots from the frozen city aren't charging, they do no contact damage. Yes, GG can kill them like that)




Impact Wrists
Corpses Fly and Hit Harder

When I first saw this mutation, I had no clue what did it do. Early Game, it really doesnt do that much most of the time. But Mid Game and onwards(4-1 is the start of mid-game), its amazing.

Every enemy you kill has his corpse thrown around and can hit another enemy and even kill it like that.


Scarier Face
Reduces Enemy HP

All enemies have 20% less HP, rounded up.




Sharp Teeth
Being Hit Deals Damage to All Enemies On Screen
Just what the description says, only that now(Update 96) it seems to have a small delay before hitting enemies.

Honestly, I prefer Gamma Guts, but this in no way means that this mutation is bad.

Back Muscle
Increases Ammo Max

Useful for pretty much every situation, as it'll give all of the weapon types more ammo to use. Energy and Bolts for example go from 55 to 99. And Bullets go to 555. The importance of it depends on your ammo needs in that specific run where you see it.




Bloodlust
Some kills regenerate HP

The exact chance is 7.69%.
Its the same as Back Muscle in terms of usefulness. Always great but sometimes a lot more needed than usual.



Eagle Eyes
Increased accuracy

This mutation is commonly hated on. The main reason is that during late game spread is needed, and this mutation pretty much destroys spread weapons.

But it focuses all of the projectiles a Triple Machinegun for example shoots into 1 the same shot, meaning its more accurate. And basically turns all the shotguns into the Eraser weapon. It can be fun, but if you want spread/late game survivability, you shouldnt take it.



Extra Feet
Increased Movement Speed and Walk Normally on All Surfaces

Originally, this mutation only gave you more speed. And that still is the main reason to take it, as out of the 2 surfaces that will affect your movement speed in the game(The Ice in Frozen City and the Spiderwebs in Crystal Caves), only one of them is actually an issue(The webs).

Though if you're planning on going to the Cursed Crystal Caves, TAKE THIS. There's a possible layout that pops every some time where the entire level is covered in cobwebs, and considering the dodging you need to do to dodge teleporting crystals and splitting spiders that are pretty fast moving, you do not want to be there without Extra Feet.



Last Wish
Full HP and Some Ammo

It really isnt as good as you will first think. You have 8 HP usually, so 3 Medkits and you can even be saved from 2 HP.

I am not saying its horrible, I sometimes pick it, but pretty much only in hopeless cases.

This mutation also gives you access to a secret area of the Frozen City, and will revive Chicken from being headless and dying while inside a portal, as well as restore here to her original max HP.



Lucky Shot
Some Kills Restore Ammo

The catch with this mutation is that the ammo some kills can give you is not necesserily for the weapons you currently have.
Its still is nice in some cases, but if your ammo needs depend on it...I would pick something else.
(The chance for kills to restore ammo is 11.11% by the way)



Open Mind
Extra Chest Drop

Its not game breaking, but it can do some cool things. It can give you an extra weapon chest, ammo chest, HP chest or Radiation Canister each level.




Patience
Mutate Later

If you dont like your current mutation options you can choose from, you can take Patience, skip a mutation this level, and in the end of the next one you will have 4 new options to choose from.



Plutonium Hunger
Pickups Attract From Further

You attract rads(exp) to you all the time, and this mutation makes you attract them from a longer distance, on top of making the other pickups(ammo, medkits) be attracted to you a bit.

Not a must, but defiently helpful.



Rabbit Paw
More HP and Ammo Drops

Increases the chances of enemies dropping stuff by 40%.

Helpful for obvious reasons.



Second Stomach
HP Drops Restore More HP
Doubles the amount of HP medkits and Large Medkits give you, as well as the amount of HP Robot can gain from eating weapons.




Stress
Shoots Faster on Lower HP
It shoots faster based on how much HP do you have out of your max HP, and can only reach 76%, unless you're Chicken and Headless, which puts you at 80% faster attack rate.

Its not game breaking, but the faster firerate is pretty helpful.


Throne Butt
Different Effects for Each Character
Every character has a throne butt that modifies their active ability.
I will only list the 12 selectable characters' throne butts, and not the April Fools Big Dog or any of the Secret characters.

Fish: Infinite Roll. Good for every reason the normal roll is good, but more so.

Crystal: Creates a shield, has shorter shielding time, and when the time is over, teleports into where your cursor is. Can be good, but a good amount of players and me can't control this very well.

Eyes: Pulls enemies MUCH faster and pushes bullets away in a stronger way. Is helpful, but also a bit risky.

Melting: Blows corpses into 3 blood explosions instead of 1. Is amazing.

Plant: Snare finishes off enemies who are under 33% HP. Can be helpful for enemies with more HP.

YV: Replaces Pop Pop(which shoots 2 projectiles of the weapon you're holding in 1 hit but has a bit longer reload time) with Braap, which shoots 4 projectiles, and takes the normal reload time of 1 shot, multiplied by 3. Worth it.

Steroids: Firing a weapon has a chance to give ammo to the other weapon you're holding. Will help any Steroids run a lot.

Robot: Adds additional health/ammo when eating a weapon. Is helpful, but nothing too special.

Chicken: Thrown weapons pierce through enemies. No real comment as I'm one of the people who hate Chicken's active.

Rebel: Higher ally firerate. So helpful. Almost every good Rebel build should have it.

Horror: The beam gets wider and faster, and every 1.5 seconds you shoot it, you get 1 HP back. Very helpful, especially because Late Game you're gonna be spamming the beam everywhere.

Rogue: Portal Strike does more damage. Pretty helpful.




Trigger Fingers
Kills Reduce Reload Time
Y'know all of the weapons that do great Damage but have a slow reload time? (Shovel, Sledghammer, Super Plasma Cannon,etc.) Well, Trigger Fingers makes every kill you get with them to reduce the reload time by 40%. And with their great damage, kills are not hard to get.

Super helpful.

Upon reaching Level 10, you will be able to pick a mutation out of 2(or 3 if you're Horror) mutations specific to your character, called Ultras. After that you will not be able to get any more mutations.

I did not talk about them because I didnt take every ultra yet and dont really know what to say about the ultras I never used that'll be helpful.

So instead, here's a Yeti asking for forgiveness.