I've been looking around for a reference to defending. I see it mentioned on a couple of cards, and once in the rule book:

A player may spend (discard) a Support Token from one of his Space Marines when it is attaching or defending to reroll a die that was just rolled.

What is the defending? Does it mean when you are attacking the Space Marine with the genestealer? I would like some clarification if anyone can offer it. Thanks.

Yes, defense rolls are the ones you do in the Genestealer Attack Phase. And remember you have to roll over the number of GS in the swarm, equal or less = dead.

Thank you for the input. It is weird that they refer to it as two different things. Makes things a little complicated in my mind. I will have to do a solo run through, or two, before getting together with some friends to play next weekend. Thanks again Dam.

I have a little follow up question - can I reroll even when the swarm misses my marine? I'm of course talking about Lorenzo. When you line him up against 4 stealers with counter attack card and bunch of support tokens ready...and he rolls 5 with its first try.

I play it that Lorenzo may spend a support token and continue rolling hoping for skulls, but I'm not absolutely sure it's correct.

 I play as if you can re-roll a success to get a better success, but I don't have the exact wording that would allow or deny it.

Sid Loman said:

I play this way too. It should be legit to reroll even when they miss, as the rules state that you may reroll a valid attack or defense roll by spending a support token. Just be sure Lorenzo is facing the correct direction is all when defending.