Board Thread:Warhammer 40k General Discussion/@comment-6078851-20130130174042/@comment-3258762-20130316141630

Gene-Seed is an organ which allows a human body to accept the modified organs needed to become an Adeptus Astartes, it is a distilled form of the Primarchs genes, and more importantly, something only the Emperor (and possibly Fabius Bile) knew how to make and something he never made of himself.

Like I said: It doesn't matter that they have pure gene-seed, because they would still be susceptible to corruption. The only beings in the universe completely immune are pariahs, and after that comes the Adeptus Custodes since they are constantly so close to the Emperor. If 9 out of 18 Primarchs were corrupted, why would the gene-seed of one primarch mean the AL cannot be corrupted?

The Grey Knights are Astartes. Being implanted with the Gene-Seed of a Primarch is dangerous enough, taking a pure sample of a Primarch (as Rafen did) is extremely dangerous and can nearly kill an astartes. A Primarch is a much weaker version of the Emperor, as we can see when a daemonically empowered Horus lasted only a second against the Emperor when he unleashed his powers. Spear, a Pariah, was literally ripped apart and turned inside out by a single drop of blood, the only thing keeping him alive his Daemonskin. Now imagine this.

Blood of the Emperor rips a normal human apart. Blood of a Primarch almost kills a Space Marine. Blood of the Emperor would then, naturally, highly damage a Space Marine, almost to the point of death IF they survive. Now, replace that one drop of blood with an organ. This is the comparison I draw

You drink alcohol first, it makes you feel bad after too much. Then you drink 50% alcohol, it makes you feel VERY BAD. Then you drink gasoline. You need medical attention immediately. Then you drink plutonium mixed with acid and nitroglycerin. That is probably what you would get from being implanted with the Emperors Gene-Seed, if it existed. And, without it, the Grey Knights cannot be Astartes. So, either they are not directly decended from the Emperor, or they are not Astartes at all.