Mutation Jacket (or Mutation Armor) is a special item for Mutation Mode and variants.
When equipped, soldiers have one-hit protection against the Mutants (The small Nano Armor icon displays and will disappears upon first hit), and as a Mutant, they will have 1000 armor (This works together with normal armor, so a Mutant can have 1100 armor in total). Armor for mutants won't regenerate, but it will come back when the mutants respawn. This is a very useful item as the soldiers will have more chance to escape from the Mutants, and the Mutants can live longer to get to the Soldiers.
The Hero comes equipped with his own Mutation vest, and if players already have Mutation vest equipped, they will be protected against 2 hit (A small x2 icon will display on the Nano Armor icon).
When becoming the Commando, the Mutant Vest can take up an additional strike, allowing the Commando to survive longer. This protection is useful against normal attack from the mutants, but especially efective against Xeno since its RMB attack will deal only 3000 HP, nullifying first strike of two. In some localisations It's also available in Supply Box, but if you have one, it isn't stacked.
Beside equipping one Mutation Vest from the inventory, players can obtain it through Supply Box. The advantage is similar to Hero Mode, soldiers can stack Mutation Vest up to 3, which can help them survive longer for the battle. But due to mode rules if soldier got hit in mutant frontal attack and mutant mutated, it'll be impossible to escape, 'cause mutant level up grants invisibility for a second, allowing mutant to strike again and again.