Marked

Marked is a perk in the Fallout: New Vegas add-on Lonesome Road.

Obtaining the perk
In order to obtain the perk, you must kill any three of the following named Marked Men: Blister, Bonesaw, Blade and Beast. You should be able to obtain this easily during the Lonesome Road main quest, unless you are attempting to use stealth to get through it.

Effects
This perk gives you a bonus of +10% Damage resistance* and +10% damage against Marked Men.

Bugs
It actually gives you +10% Damage Threshold, not Damage Resistance, but it will not work either way. This is because the condition is broken (instead of checking if the target is a marked man, the perk checks if the target's weapon is a marked man, which obviously can never return a meaningful value).