"Early in the 21st Century, THE TYRELL CORPORATION advanced Robot evolution into the NEXUS phase -- a being virtually identical to a human -- known as a Replicant.
The NEXUS 6 Replicants were superior in strength and agility, and at least equal in intelligence, to the genetic engineers who created them.
Replicants were used Off-world as slave labor, in the hazardous exploration and colonization of other planets.
After a bloody mutiny by a NEXUS 6 combat team in an Off-world colony, Replicants were declared illegal on earth -- under penalty of death.
Special police squads -- BLADE RUNNER UNITS -- had orders to shoot to kill, upon detection, any trespassing Replicant.

This was not called execution.
It was celled retirement."