DNA resequencing is the region specific manipulation of the genetic structure of an individual. DNA resequencing for any reason other than repairing serious birth defects is illegal in the Federation.
In 2154, the Klingons used embryos from genetically engineered Human Augments to resequence a number of test subjects. Unfortunately, there were some unanticipated side effects, including the dissolving of their cranial ridges. Augment DNA was found to be more aggressive than the Klingons realized; nevertheless, the test subjects became stronger and more intelligent before their neural pathways unexpectedly started to degrade, and they died in agony (see Klingon Augments). (ENT: "Affliction")
In 2348, Julian Bashir, a Human from Earth, illegally received a form of DNA resequencing on Adigeon Prime known technically as Accelerated critical neural pathway formation. These "treatments" accelerated the growth of neuronal networks in his cerebral cortex, enhancing his mental abilities and allowing his intelligence quotient to jump five points a day for over two weeks. Further "treatments" led to improvements in his hand-eye coordination, reflexes, vision, stamina, height and weight. (DS9: "Doctor Bashir, I Presume")
Unlike the Bashir family, other Human families who attempted to have their children's DNA illegally resequenced did not find doctors as competent. Many suffered unintended side-effects, including the children Jack, Patrick, Lauren and Sarina Douglas. By the time they were five or six years old, their parents had to come forward and admit that they'd broken the law so their children could get treatment. There was not much the doctors at the Institute could do for them, since cases like theirs were so rare there was no standard treatment. (DS9: "Statistical Probabilities")
In the case of Sarina Douglas, her parents had her cerebral cortex genetically enhanced in order to accelerate the rate at which she processed information. Unfortunately, a complication in the procedure occurred, and her visual and auditory systems were out of sync; thus, she was unable to channel stimuli into her cortex fast enough, and therefore unable to focus on what was going on around her. This was corrected by Dr. Bashir in 2375, when he sped up her sensory processes by stimulating the growth of new synapses in her thalamus. He accomplished this by manipulating her neural proteins at the subatomic level. (DS9: "Chrysalis")
In early-2370, the Ferengi Bok used genetic resequencing to alter the DNA of a Human named Jason Vigo in order to convince Jean-Luc Picard that Vigo was his son. Bok's technique, however, was flawed and Vigo began to suffer from a neurological disorder called Forrester-Trent Syndrome. Dr. Crusher investigated its cause and upon concluding a microcellular scan, she discovered Bok's deceit. After a series of treatments to Vigo, Dr. Crusher was able to completely reverse the effects of Bok's botched attempt. (TNG: "Bloodlines")
In 2377, Icheb proposed genetic resequencing to The Doctor, as a means of regulating his body's Borg implants without having to rely on his cortical node. Icheb wished to donate his cortical node to Seven of Nine in order to save her life. (VOY: "Imperfection")