Marked

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

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

Note
It actually gives you +10% Damage Threshold, not Damage Resistance, but in will not work, 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).