Improved Uncanny Dodge (Ex)

At 5th level and higher, an assassin can no longer be flanked. This defense denies a rogue the ability to sneak attack the assassin by flanking him, unless the attacker has at least four more rogue levels than the target has assassin levels.

If a character already has uncanny dodge from another class, the levels from the classes that grant uncanny dodge stack to determine the minimum rogue level required to flank the character.