Regenerative fusion was an experimental process developed by Ma'bor Jetrel between 2355 and 2371. Dr. Jetrel believed that people who had been vaporized by the Metreon Cascade on the moon Rinax could be restored by using a starship's transporters to lock on to particular DNA sequences (based on those in a victim's medical records) in the metreon cloud surrounding the moon. By isolating those particular DNA fragments in the cloud the transporters, in theory, would then be able to rematerialise one of the victims to their previous state.
When Dr. Jetrel put his theory into practice, with the help of Captain Janeway and Lt. Tuvok, it failed because the DNA fragments were too severely fragmented to be reconstituted. (VOY: "Jetrel")