The primary flaw in the Wolves gene-seed is the Curse of the Wulfen, which risks transforming the Marine into a feral beast that cannot be controlled. This risk presents itself when Aspirant's are first implanted with the gene-seed before their Test of Morkai, but even those who manage to control the instability are never entirely free of it. The struggle against the Wulfen can last throughout the Space Wolf's life.[8f]