Tyrell: The facts of life: To make an alteration in the evolvement of an organic life system is fatal. A coding sequence cannot be revised once it's been established.
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 sulfonate is an alkylating agent and a potent mutagen. It created a virus so lethal the subject was dead before he left the table.
Roy: Then a repressor 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 a mutation and you've got a virus again. But 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. Look at you. You're the prodigal son. You're quite a prize!
Roy: I've done questionable things.
Tyrell: Also extraordinary things. Revel in your time!
Roy: Nothing the god of biomechanics wouldn't let you in heaven for.