Board Thread:Warhammer 40k General Discussion/@comment-58.178.141.62-20140405080514/@comment-71.163.66.194-20140407154413

"Canonically" speaking, the fluff explicitly stated that the Imperium has traitor gene seed in storage. It's just forbidden to use (although I imagine anyone making a space marine chapter from traitor gene seed probably doesn't care about or has the authority to override those laws in the first place)

That said, there was more than enough Thousand Sons sent away by Magnus to create an entire chapter of marines anyways, if I recall correctly, so there probably wasn't any need to access the storage stocks.