An artificial retrovirus that targets and kills only those persons with specific gene sequences.

This assassin virus project was led by Richard Ames under the authority of President George Sears. It was developed by a team of scientists at ATGC Corporation led by Naomi Hunter, who created the genetic identification program. Under orders from CIA director Jim Houseman, the virus was distributed by using Solid Snake as a vector during the Shadow Moses Incident. The virus was named "FOXDIE" since it killed members of FOXHOUND.

FOXDIE is composed of a fiber structure developed based on DDS (drug delivery system) technology. It multiples and lies dormant within the vector's body. It infects its targets through airborne transmission. Once inside the target's body, it begins to produce TNF-epsilon using the target's macrophage cells. TNF-epsilon is a peptide in the cytokine family that triggers cell death. It is normally rendered harmless and naturally excreted unless ordered to activate by macrophages. When activated, however, it passes through the bloodstream to the heart, where it binds to receptors on cardiac muscle cells. This stimulates the cells and causes sudden apoptosis. The target then goes into cardiac arrest and dies.

In the Shadow Moses Incident, FOXDIE was programmed to target the hostage Kenneth Baker as well as Liquid Snake and the other members of FOXHOUND. (Donald Anderson was not targeted because he was a good friend of Jim Houseman and was still needed to bring back the test data from Shadow Moses.) However, Naomi Hunter reconfigured the genetic identification program just before the mission to change the targets. The new target added by Hunter was Solid Snake. It was believed that Naomi did this because she held a personal grudge against Snake, but exactly how she changed the program remained unclear. There were three confirmed deaths on Shadow Moses from FOXDIE: Decoy Octopus, Kenneth Baker, and Liquid Snake.