Final answer:
Scientists refer to ubiquitin as the "death tag" for proteins in eukaryotes, marking them for degradation by the proteasome.
Step-by-step explanation:
The "death tag" for proteins in eukaryotes is called ubiquitin. The process involves the addition of an ubiquitin group to a protein, which then marks that protein for degradation.
This ubiquitous molecule indicates that the protein's lifespan has reached its end. Proteins tagged with ubiquitin are subsequently transported to the proteasome, which is an organelle that specializes in degrading proteins, effectively controlling gene expression by altering protein longevity.