@gentoobro @Groomschild @Terry The problem lies in that fact that genes don't exist independently of one another. The gene that prevents you from re-growing severed limbs might also be the same gene that prevents you from developing all kinds of cancer. Almost every trait we have is polygenic i.e. influenced by multiple genes that interact with one another. If you slightly adjust human genetic code to allow re-growth of limbs, you don't really have any way to predict what the after effects might be. You could get a tumor, cancer, skin disease, incurable infections etc. Even a small genetic tweak could lead to unintended consequences, and there's no real way to predict those as most of the data emerges ad-hoc or after the fact.