A genetic tag was a sub-molecular device used by the Srivani in their research. They were implanted on an individual's DNA 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 USS Voyager and planted 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")