From the Roy/Tyrell conversation:
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 for 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.
30 years and several revolutions in genetics and genomics have passed since then. I believe that the line "A coding sequence cannot be revised once it's been established" has been proven false since then. But in other ways we've gotten closer, although we're at this point trying to understand things, not so much not modifying them, and that's the deepest they get into the science of it.
