In eukaryotes, a processed mRNA must exit the nucleus before the translation step of gene expression can occur. The mRNA migrates to the cytoplasm through one of a few thousand nuclear pores, large protein complexes that span both layers of the nuclear envelope and regulate the flow of macromolecules in and out of the nucleus. Once the mRNA is in the cytoplasm, there are multiple opportunities for gene regulation at the levels of mRNA stability, translation, and protein activity (see Figs. 19.1d–19.1f).