The Hybrid Breeding Program was a Terran Dominion research program, used to develop protoss/zerg hybrids for use as weapons. These breeding programs were done in conjunction with programs from the Moebius Foundation.
The program had been established by the Second Great War, and was headed by Emil Narud. The hybrids were created through splicing zerg and protoss DNA. This was technology that was beyond the scope of terran technology. Mengsk believed that the hybrids would follow his orders and could be used as weapons, though Alexei Stukov had doubts as to this. Valerian Mengsk had knowledge of the involvement of his Moebius Foundation in the creation of the hybrid, but turned a blind eye to it.
After obtaining classified documents during a raid on Korhal, Raynor's Raiders were led to the Castanar facility. Here they found hybrids being created, with advanced weaponry being tested on them, as well as experiments on zerg. The facility was destroyed after a nearly invincible hybrid was released, and the facility's fusion core destroyed. Jim Raynor was left to wonder how many other similar labs the Dominion was operating.
In the last days of the Second Great War, Sarah Kerrigan brought the Swarm to Ketill at the request of Stukov, in order to destroy the base from which the program operated. Izsha believed that if the hybrids were used when the Swarm attacked Korhal, their attack would fail. The Swarm overran the Dominion forces stationed on the base's exterior and entered its depths, where they encountered Emil Narud, a scientist who had also created hybrids through his position in the Moebius Foundation. Hybrid dominators were released, but destroyed. The platform was laid waste and Narud defeated.
With the Dominion's Hybrid Breeding Program destroyed, only the Moebius Foundation's program remained intact. As the hybrids there began to awaken, they enthralled of the scientists of the foundation and security personnel of Moebius Corps, and began to destroy planets of the sector over the course of the End War. However, these facilities too were destroyed when the Daelaam attacked their central breeding operations on Revanscar, ending all hybrid breeding operations throughout the sector.