During the Horus Heresy Alpharius (it may or may not have actually been him) stole the improved Gene seed Corax developed to create his Space Marines MKII. He then left a tainted gene seed in place mutating the next batch.
He was then shown giving a different gene seed sample to Fabius Bile, not wanting him to have the improved seed for himself.
There have been several cases of writers starting plot lines in the Horus Heresy books that tie into or set up story ideas in books set in the 41st millennium.
Has this improved gene seed reappeared in any other 40K fiction or background information?