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")
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")