A genetic tag was a sub-molecular technology used by Srivani scientists in their research. They were implanted on an individual's DNA base pair and caused rapid, selective mutations. The tags were slightly out of phase, and so were only detectable with an electron resonance scanner.
In 2374, a Srivani research team infiltrated the USS Voyager and planted tags on the DNA of the crew. Chakotay and Neelix were the first to experience the mutations. The Doctor discovered the tags and believed that a neuroleptic shock might disable them, breaking the Srivani's hold on the crew. However, he and Seven of Nine were unable to initiate the shock due to Srivani interference.
After the Srivani were driven off the ship when Captain Kathryn Janeway set the Voyager to fly between binary pulsars, The Doctor was able to neutralize the devices without hindrance. (VOY: "Scientific Method")