Gene-bombs are weapons used by the Xorda, that are designed to cause the devolution of a species on a planet.
Gene-bombs were used during the Xorda's attack on Earth which resulted in the devolution of the human race into the four-fingered Overlander species (although it was apparently supposed to devolve humans much further back, to pre-intelligence). It also resulted in the rapid evolution of the animal kingdom, blending their DNA with that of humans to form the Mobian species. Additionally, the gene-bombs mixed with various beryl deposits beneath the planet's surface, resulting in an environmental cataclysm known in Echidna theology as the first Day of Fury. (StH: #124, #125, #148)