A genetic tag is a sub-molecular device used by the Srivani in their research. They are implanted on an individual's DNA and cause rapid, selective mutations. The tags are slightly out of phase, and so are only detectable with an electron resonance scanner.
In 2374, a Srivani research team infiltrated USS Voyager and planted the tags on 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 Voyager to fly between binary pulsars, The Doctor was able to neutralize the devices without hindrance. (VOY: "Scientific Method")