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Shortly after Kerrigan defeated the United Earth Directorate forces on Korhal and retreated to Tarsonis, the UED responded with a counterattack, sending a pair of slave broods, commanded by their Science Corps, to attack Kerrigan's forces on Tarsonis.
The genetic engineers experimented on passive zerg, and security guards and pyrotechnic engineers were assigned to protect them. They also healed UED troops during the battle. However, Kerrigan's forces defeated them, and the formerly passive zerg rampaged across the battlefield.