The indirect targeting of mRNA is essential to translating ribosome affinity purification (TRAP). By attaching an affinity tag, such as enhanced GFP (EGFP), to the large ribosomal subunit protein L10a, indirect labeling of mRNA can be accomplished. The EGFP-L10a transgene is targeted to express in the desired cell type using the right genetic components. While translating polysomes from targeted cells have EGFP tags, those from non-targeted cells do not.
For more: https://ribosome.creative-biolabs.com/translating-ribosome-affinity-purification-trap.htm