Genetic engineering is the artificial alteration of a biological entity's DNA.
Genetic engineering is considered a controversial subject by several races. Notably among Humans, due to genetic engineering bringing about one of humanity's greatest tyrants, Khan Noonien Singh and spurring the Eugenics Wars. As such the practice was outlawed in the Federation at least well into the 24th century. (TOS: "Space Seed"; DS9: "Doctor Bashir, I Presume") Genetic engineering leading to the four gender arrangement, however, saved the Andorian race from extinction, so most Andorians are baffled that humans are so averse to the idea. (Star Trek: Lambda Paz Background Essays, Worlds of Deep Space Nine Expanded Universe)
Scott Fack, Garrett, Anne de Gaillimhe and several others were genetically modified by the M'Tar to serve as Liaisons and Links in The Great War. (Star Trek: The Cantabrian Expeditions: "Catalyst, Part One", "White Flag")