Roy: Why not?
Tyrell: Because by the second day of incubation, any cells that have undergone reversion mutations give rise to revertant colonies like rats leaving a sinking ship. Then the ship sinks.
Roy: What about EMS recombination?
Tyrell: We've already tried it. Ethyl methane sulfanate as an alkalating agent and potent mutagen. It created a virus so lethal the subject was dead before he left the table.
Roy: Then a repressive protein that blocks the operating cells...
Tyrell: Wouldn't obstruct replication, but it does give rise to an error in replication so that the newly formed DNA strand carries the mutation and you've got a virus again. But, uh, this-- all of this is academic. You were made as well as we could make you.
Roy: But not to last.
Tyrell: The light that burns twice as bright burns half as long. And you have burned so very very brightly, Roy.
...
Roy: I've seen things you people wouldn't believe. Attack ships on fire off the shoulder of Orion. I watched C-beams glitter in the darkness at Tan Hauser Gate. All those moments will be lost in time like tears in rain.
Time to die...
Wasteful.