The facts of life... to make an alteration in the evolvement of an organic life system is...

Tyrell: [Tyrell explains to Roy why he can't extend his lifespan] 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. Batty: Why not? Tyrell: Because by the second day of incubation, any cells that have undergone reversion mutation give rise to revertant colonies, like rats leaving a sinking ship; then the ship... sinks. Batty: What about EMS-3 recombination? Tyrell: We've already tried it - ethyl, methane, sulfinate as an alkylating agent and potent mutagen; it created a virus so lethal the subject was dead before it even left the table. Batty: Then a repressor protein, that would block 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 with it 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. Batty: 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! Batty: I've done... questionable things. Tyrell: Also extraordinary things; revel in your time. Batty: Nothing the God of biomechanics wouldn't let you into heaven for.