From "Star Trek: Deep Space Nine: Doctor Bashir, I Presume? (#5.16)"
Two hundred years ago, we tried to improve the species through DNA resequencing. And what did we get for our troubles? The Eugenics Wars. For every Julian Bashir that can be created, there's a Khan Singh waiting in the wings, a superhuman, whose ambition and thirst for power have been enhanced along with his intellect. The law against genetic engineering provided a firewall against such men. And it's my job to keep that firewall intact.