"It is not the strongest of the species that survives, nor the most intelligent that survives. It is the one that is the most adaptable to change." - Charles Darwin

This guide assumes you are already familiar with how the Gene Mods work and how they are obtained.

Not all soldiers are equal; some classes specialize in certain areas. Because of this, certain soldiers enjoy priority over others in receiving modifications since they can make better use of it than others. The priority system also makes sense given that Meld and cash are often scarce especially in higher difficulties.

This guide is not Gospel truth nor is it applicable in every possible situation. If you have your own playstyle that works for you, you should play your way and have fun.

“When you decide to attack, keep calm and dash in quickly, forestalling the enemy...attack with a feeling of constantly crushing the enemy, from first to last.” - Miyamoto Musashi

The skill builds may vary but the Assault has two distinct specialities. First, they are the very best at close-quarters combat with the Shotgun, Close Quarters Specialist and Close and Personal. Secondly, they are excellent frontline troopers; the best at scouting ahead and flanking the enemy with Run and Gun.

The Assault excels at fighting; whether offensive or defensive. They have several skills you can take to improve their survivability such as Tactical Sense, Lightning Reflexes, Extra Conditioning and Resilience. This means they are more difficult to kill and thus, mods won't be wasted.

The Assault enjoys the highest priority since he benefits the most from gene mods.

- Suggested Mods -

Neural Damping The Assault is one of the most lethal soldiers in your squad, especially when armed with a shotgun. For this reason, Neural Damping is the best since you do not want your Assault, under any circumstance, to be mind-controlled and be made to wreak havoc on the rest of your squad. The immunity to panic and the additional will also greatly helps Assaults with low will.

+ + +
Hyper Reactive Pupils + Rapid Fire + Close and Personal + Close Combat Specialist
The best choice in almost all cases. The Assault's Rapid Fire immediately benefits from this mod if the first shot misses. Free shots like Close and Personal and Close Combat Specialist can turn a miss into an added benefit.

+
Mimetic Skin + Run and Gun
This gene mod really fits the Assault's playstyle. Since the Assault is the first to scout ahead, this mod allows him to do so safely. The Assault is one of the best classes to modify with Mimetic Skin since Run and Gun opens up a lot of options. The Assault can, for example, move to Heavy Cover, spot the enemy, take Combat Drugs while the rest of the squad engages the enemy. Then, in the next turn, storm the "safe" enemy positions with a devastating flanking attack with Run and Gun.

Warning: This mod conflicts with Close Combat Specialist. If an alien moves too close to the Assault, he will fire and come out of the invisibility from Mimetic Skin. This will make him vulnerable to follow-up attacks from aliens who haven't moved yet.

+ + +
Secondary Heart + Adaptive Bone Marrow + Extra Conditioning + Resilience
These mods synergize well with the Assault's skills and give a boost to his innate survivability. The Assault can make great use of the Adaptive Bone Marrow with the high base health he can acquire from Armor and Extra Conditioning. The Resilience skill prevents sudden spikes in damage from critical hits. All these combine to make damage much more manageable and allows the Assault to soak up a bit of damage (if necessary) and always be in a state or combat readiness - if given some time to heal.

The Secondary Heart goes well with this set-up. It eliminates the will penalty from the critical wounds Assaults tend to receive regularly. Having your Assault live on only his base health is also very risky. Worst case scenario? At least the Secondary Heart allows you to stabilize your Assault instead of death. The Adaptive Bone Marrow (with Rapid Recovery) will help your Assault recover in time for the next mission should that happen.

If your Support has Revive then that's a great combo too, since the Assault can heal up to full base health by himself.

+
Muscle Fiber Density + Run and Gun
The Assault excels in making bold, aggressive movements in the battlefield thanks to Run and Gun. Run and Gun allows the Assault to quickly take advantage of terrain to get flanking shots on the enemy. Muscle Fiber Density greatly improves his range of movement making flanking easier or, if flanking is not possible, take advantage of high ground for the added aim bonus. This is great on Riflemen Assaults.

- Situational Mods -

Neural Feedback - Preventing your Assault from being mind-controlled is top priority so Neural Damping is the safer bet. However, there are instances when Neural Feedback is an acceptable choice. If your Assault has exceptionally high will, whether from the Iron Will uprade, medals or items like Mind Shield, Neural Feedback makes sense because your Assault may resist the psi attack and deal a lot of damage in return.

Depth Perception - This mod makes sense only if you took Muscle Fiber Density or armor with a grappling hook. Even then, the situations when the high ground really matters are few and far between. This mod is useless on Shotgun Specialists who need to be right next to the enemy anyway.

Adrenal Neurosympathy - If you don't care for the Secondary Heart, this mod is best on Assaults since they have the killing power to trigger Combat Rush for their squadmates. However, here are some things to consider.

Shotgun Specialists may not be the best for this mod for two reasons; first, they tend to fight deep into the enemy positions and their squadmates may not have line of sight on the Assault to receive the Combat Rush buff. Secondly, the added aim from Combat Rush is moot in close quarters and the critical chance isn't that powerful since the shotgun already has a high critical chance, not to mention flanking bonuses.

Riflemen Assaults perform spectacularly with Combat Rush. The added aim, critical chance and movement from Combat Rush fit their aggresive playstyle perfectly. The real question is whether the Assault should give up the Secondary Heart to carry this mod or just have another squadmate carry this mod since it's really only Combat Rush that matters. You decide.

Bioelectric Skin - Look, we all know Mimetic Skin is awesome but sometimes, Mimetic Skin is too costly or, for one reason or another, not an option. In which case, Bioelectric Skin also works best on the Assault since the Assault often does the scouting anyway.

Shotgun Specialists benefit from this mod since it's usually them who stack up behind a doorway before breaching a room. Bioelectric Skin can detect the threats and their positions before storming them. The immunity from strangulation is also a bonus since Shotgun Specialists are good at Seeker-peeling duties.

"God fights on the side with the best artillery." - Napoleon Bonaparte

The Heavy is XCOM's Demolitions Expert. The Heavy is the best in dealing massive area-of-effect damage using the Rocket Launcher and grenades. The Heavy can also be a Gunner; using the LMG to supress the enemy or tear them to pieces with a hail of Overwatch fire.

Like the Assault, the Heavy needs to be at the frontlines to do his job. The Heavy has mobility issues since he needs to "set-up" before using the Rocket Launcher. Survivability is also a concern since the Heavy is often in the firing line.

The Heavy is second to the Assault in Gene Mod priority.

- Suggested Mods -

or
Neural Damping or Neural Feedback
You will always want some kind of brain mod for your Heavy since he will be in the frontlines and will be susceptible to psi attacks.

The same rules as in the case of an Assault applies. Neural Damping is highly recommended. You do not want your Heavy to be mind-controlled and have all that weaponry turned against you. The immunity to panic helps Heavies with low will.

Otherwise, if your Heavy has very high will or you dont mind the slight chance of your own explosives used against you, get the Neural Feedback mod.

+ +
Hyper Reactive Pupils + Bullet Storm + Rapid Reaction
The best choice for the Heavy especially for a Gunner. It synergizes very well the skills that allow you to fire twice: Bullet Storm and Rapid Reaction. If the first shot misses, you get a bonus on the second. The Heavy fires his gun a lot but has the lowest aim of all the classes so this mod mitigates that weakness by turning misses into bonuses.

+ +
Secondary Heart + Adaptive Bone Marrow + Will to Survive
The Heavy is a frontline soldier and boosting his survivability is never a bad idea. The Secondary Heart and Adaptive Bone Marrow combo synergizes with Will to Survive in making damage manageable allowing you to heal up. It doesn't work as well as the Assault but it works nontheless. Be warned though, that Will to Survive is useless when flanked and the Heavy is still vulnerable to the occassional critical shot unlike an Assault with Resilience.

Since the Heavy tends to get damaged a lot, the Secondary Heart should be handy in removing the will penalty from critical wounds. In higher difficulties, you will be reliant on Heavies and their rockets to even the odds. The Adaptive Bone Marrow is crucial in making sure Heavies are always available for the next mission.

+
Mimetic Skin + Fire Rocket
Heavies suffer from the need to be in position first before they can launch their rockets. Gunners don't suffer as much, but the bottom line is that Heavies need to be in a good position at all times to find the range for their rockets or to cover the area in Overwatch fire. Mimetic Skin removes that weakness and gives the Heavy the luxury of getting into position in safety without being seen.

Here's a nifty trick; Provided the Heavy hasn't been seen, you can move him far and way up to heavy cover deep in enemy territory and leave him cloaked there. You can then move your squad up to engage the enemy. When the aliens take cover, the Heavy can ambush them and launch a rocket to destroy them and their cover.

Whether Mimetic Skin works better on a Heavy or an Assault depends on the player and who his "MVP" is. Still, Heavies can get a lot of benefits from this mod.

- Situational Mods -

Depth Perception - The Heavy isn't a very mobile class; other classes are more adept at exploiting high ground. Without Run and Gun, Sprinter or Damn Good Ground, you need the Muscle Fiber Density mod or armor with a grappling hook.

Remember that the Heavy has poor aim. You will probably miss more often than get a chance to use high ground. Better to have Hyper Reactive Pupils, which will benefit you all the time, than Depth Perception, which will benefit you some of the time.

Adrenal Neurosympathy - It's a mixed bag. On the one hand, Demolitions Heavies can trigger Combat Rush easily, but firing rockets don't benefit from the aim and critical chance bonuses of Combat Rush and if you had to use a rocket, chances are that it won't take much to finish the job anyway.

On the other hand, Gunner Heavies may not have the killing power since they're usually supressing or going into Overwatch in the hopes of getting a double shot from Rapid Reaction. They certainly benefit from Combat Rush though; two Bullet Storm shots while under Combat Rush is pretty fun.

Survivability is often more important but if you don't care for the Secondary Heart, get Adrenal Neurosympathy for the Heavy and watch him kill things.

Bioelectric Skin - A Heavy could, theoretically, detect aliens behind a wall or on the roof and fire a rocket at them. The immunity to strangulation is nice. The question though, is whether the Heavy should be the one to carry this mod? You decide.

Muscle Fiber Density - It's situational. It helps the Heavy's mobility but the Heavy is not meant to be a class that zips through the battlefield getting crazy flank shots like a Run and Gun Assault or have any skills which provide high ground bonuses like the Sniper's Damn Good Ground. It's good to have but perhaps the other classes can get more out of this mod.

"The only real power comes out of a long rifle." Joseph Stalin

The Sniper has two radically different styles. The first are the Squadsight Snipers who excel at dealing tremendous amounts of damage from a distance to high-value targets without exposing themselves to counterattack. The second is the Scout Sniper who uses his superior mobility to bring the power of the Sniper Rifle to bear without the drawbacks of having to set-up or being blocked by line-of-sight.

Snipers are glass cannons. They deal most of your damage but have little in the way of survivabilty. They rely on long range and high ground more than the other classes.

Snipers don't really need a lot of mods to be effective. They tend to stay far away from the battles. A few key mods will do and even then, there may be ways around their weaknesses such as S.C.O.P.E.S. or Skeleton/Ghost/Archangel Armor. As such, there's no hurry to modify them.

- Suggested Mods -

and/or +
Depth Perception and/or Muscle Fiber Density + Damn Good Ground
These mods works best with both kinds of Snipers since they ought to and should be using high ground as much as possible.

Muscle Fiber Density helps Scout Snipers since they move around a lot more and it frees them to use the heavier armors to compensate for their lack of survivability. It may not even be necessary for Squadsight Snipers since, being away from danger, they can take their time getting to high ground, could use the lighter armors with grappling hooks or use the Archangel Armor for instant high ground (overkill!).

+
Mimetic Skin + Low Profile
Talk about synergy. Mimetic Skin goes so well with Snipers, it's ridiculous. Mimetic Skin allows near total freedom of movement for the Sniper by allowing him to be invisible in any type of cover. This allows the Sniper to take the best firing position easily and in complete safety without worrying about being shot.

Scout Snipers make great use of this since there's usually only low cover at the top of buildings. Squadsight Snipers however, have no use for this at all since they don't need invisibility staying way behind the frontlines.

Those three mods are the important ones. The rest are situational or are only really relevant to the uncommon Scout Sniper.

- Situational Mods -
or
Neural Damping or Neural Feedback
Brain mods shouldn't be necessary since your Sniper should be far away from the threats leaving the other classes more at risk of psi attacks. Those classes should receive the brain mods first.

Still, if you feel the need for a brain mod, follow the general rule: If the Sniper has low will, get Neural Damping. If the Sniper has a high will score and a high chance to resist psi attacks, get Neural Feedback.

+
Hyper Reactive Pupils + Double Tap
They synergize sure, but the usefulness of the mod is dubious. This mod can be useful in the earlier months though, when Snipers still tend to miss often. However, if you're at the point when you have Double Tap, chances are your Sniper will have impressive aim already and won't miss often enough to justify this mod.

Bioelectric Skin - Mimetic Skin works better for Snipers but this is entirely up to the player. Scout Snipers have good use for it. Squadsight Snipers could, theoretically, benefit from the immunity from being singled out and strangled by a Seeker but you could just equip a Respirator Implant if that's a problem. Also, note that the Sniper's Battle Scanner ability may take the place of this mod.

and
Secondary Heart and Adaptive Bone Marrow
Squadsight Snipers have no use for these mods. They should be far away from the dangers of death and critical injury in the first place. The Secondary Heart is useful only for insurance purposes so that your "MVP" Sniper does not die a sudden death.

Scout Snipers could take these mods to improve their survivability but then again, if resources are scarce, you may have to decide if it's better on either Assaults or Heavies as they do a better job at managing damage. It depends how important you value your Scout Sniper's life. This sad choice is up to you.

+
Adrenal Neurosympathy + Executioner
Snipers are the best at killing things. Snipers with Executioner are excellent at delivering the coup de grace and can trigger Combat Rush easily enough for your team.

Scout Snipers will make better use of this mod than Squadsight snipers because Squadsight Snipers tend to be out of sight of your other soldiers such that they cannot receive Combat Rush.

The reason this is in the Situational Mod section is that the Sniper does not necessarily need to be the one to carry this mod. If you don't want to take out an insurance policy on your Sniper with the Secondary Heart, this mod is great.

"The object of war is not is not to die for your country but to make the other bastard die for his." - George S. Patton

The Support Class excels at keeping the rest of your squad alive and fighting at peak condition. The Support could be a Medic, who greatly extends the lives of your troops by healing a lot of damage and making the most out of Medkits and Smoke Grenades. The Support could also act as Fire Support, preventing damage by Supressing enemies and using Covering Fire and Overwatch to shoot enemies first before they can even attack.

The Support Class is the lowest priority for mods. Why? It's because their skills are powerful enough on their own and don't synergize as much with mods. It's mostly about the Medkits, Smoke Grenades and Overwatch. Furthermore, other classes (especially Assaults), do way better with mods than the Support could.

- Suggested Mods -

+ +
Hyper Reactive Pupils + Covering Fire + Sentinel
Like the Heavy, the Support has skills that revolve around Overwatch and potentially firing multiple shots in one turn. Overwatch shots aren't reliable so the mod should help turn any misses you get into useful bonuses.

Bioelectric Skin - This mod is suggested only because Mimetic Skin really works better on any other class. Bioelectric Skin naturally makes sense on a Support since the Support is typically in the middle of the group. this mod prevents Seekers from getting close as well as free up the Support to help squadmates being strangled.

+
Muscle Fiber Density + Sprinter
This mod allows the Support to get flanks where other classes can't. This allows you to play more aggressively like an Assault although, an Assault would probably do a better job as a flanker. Still, if you have a Fire Support, a height advantage should help make those Overwatch shots more accurate.

Other Soldiers with + +
Other Soldiers with Adaptive Bone Marrow and Secondary Heart + Revive
Revive used to be terrible in Enemy Unknown since it was very risky having your soldier fight on with low health. You needed to be really lucky to be able to Revive him in the first place. Then, you'd have to waste Medkits healing him up, suffer through the will penalty from the critical wounds and if the revived soldier drops down to zero again, it's over.

The ABM and Secondary Heart combo fixes all those problems (except for dying a second time). The soldier is easier to Revive, doesn't waste as much Medkits if you use them after all the base health heals, and there's no will penalties from critical wounds. Revive is now a very powerful ability.

- Situational Mods -

and
Neural Damping and Neural Feedback
Supports with Sprinter are often in charge of the Arc Thrower for alien capturing duties. In that case, it may be helpful to use Neural Damping to prevent being mind-controlled.

Otherwise, follow the general rule; if your Support has low will, get Neural Damping to prevent panic during combat. If your Support has high will, Neural Feedback could be more useful with the added damage.

Depth Perception - In the same vein as a Gunner Heavy, it's better to take the Hyper Reactive Pupils, especially on the Fire Support who uses all those reaction shots. It's better to have a mod that benefits you most of the time than a mod that helps only some of the time.

Mimetic Skin - This mod is always good. The Support could use this defensively to help squadmates since using the Medkit or Smoke Grenade doesn't remove the invisibility. The issue is whether this belongs to the Support class. Again, assuming resources are limited and you had to choose which class to mod with Mimetic Skin, it may be worthwhile to mod another class.

or
Secondary Heart or Adrenal Neurosympathy
Either one works really, so it's up to you. If you want more survivability, the Secondary Heart and Adaptive Bone Marrow works though keep in mind the Support has no skills to mitigate damage actually received. The Support is better at healing or preventing damage rather than taking it.

Though not as good at killing as the other classes, getting Combat Rush for your squadmates is still a great bonus if you can arrange to get it. At least one of the soldiers on your team should be carrying the Adrenal Neurosympathy mod anyway.

Adaptive Bone Marrow - If you took the Secondary Heart, you might as well take this. If not, then this mod may be better when given to either the Assault or Heavy since they're a bit sturdier and won't die as easily before this mod can be useful. Still, it's not wrong to put it on the Support per se.

"What is human warfare but just this; an effort to make the laws of God and nature take sides with one party." - Henry David Thoreau

Gene Mod Priority: Assault > Heavy > Sniper > Support

Best Users of Gene Mods

Brain Mods
Neural Damping: Assault and Heavy
Otherwise, Low Will = Neural Damping / High Will = Neural Feedback

Eye Mods
Hyper Reactive Pupils: Assault, Heavy, Support
Depth Perception: Sniper

Chest Mods
Adrenal Neurosympathy: Player's Choice
Secondary Heart: Assault, Heavy, "MVPs"

Skin Mods
Bioelectric Skin: Support
Mimetic Skin: Assault or "Scout" Snipers > Heavy

Leg Mods
Muscle Fiber Density: Assault, Sniper
Adaptive Bone Marrow: Assault > Heavy

Good luck, Commander...