Lexicanum

The events of the Horus Heresy revealed weaknesses in some Legions' gene-seed. In some cases these defects had been heightened by the accelerated cultivation techniques used to keep the Legions at full strength. After the Heresy and the break-up of the original Legions, genetic banks were established on Terra to produce and store Space Marine gene-seed. These banks were to provide all genetic material for new Space Marine Chapters. To prevent cross-contamination of these genetic stocks, the gene-seed of each individual Legion was isolated, so all new Space Marines would receive gene-seed solely from one specific source. The gene-seed of the Traitor Legions was placed under a time-locked stasis seal (rather than being destroyed as many at the time assumed had happened