Board Thread:Warhammer 40k General Discussion/@comment-6078851-20130130174042/@comment-3258762-20130316172450

I take it you mean that the DNA modifies itself to accept the new organs, because if not they would not work, but the thing is: You can almost never change the Gene-Seed. One of the biggest problems amongst the Space Marines when they have found aspirants who are worthy is that a vast majority of them are incompatible with the organs, and that the body would simply reject them. The only times when tampering has been done in great amounts led to the Cursed 21st Founding, and the only other time was when the Space Wolves tried to remove the curse of the Wulfen which met with very limited success.

The Grey Knights are far greater than any normal Space Marine, but they are not above them. They are not special because of their gene-seed, but because of their psychic powers and their rigorous training. Had they been given Gene-Seed from the Emperor, one hell of a Psyker, they would not need to take Psychic Initiates, as we can see among the Pre-Heresy Thousand Sons and the Blood Ravens, where the initiates develop Psychic powers after the implantation of the Gene-Seed.