From "Doctor Bashir I presume" as an example
BASHIR: Starfleet Medical won't see it that way. DNA resequencing for any reason other than repairing serious birth defects is illegal. Any genetically enhanced human being is barred from serving in Starfleet or practising medicine.
O'BRIEN: I don't there's been a case dealing with any of this in a hundred years. You can't be sure how they'll react.
BENNETT: I don't think so. Two hundred years ago we tried to improve the species through DNA resequencing, and what did we get for our trouble? 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 provides a firewall against such men and it's my job to keep that firewall intact.