The post-transcriptional operon provides a means of synexpression of mRNAs encoding interrelated proteins. The coordination of gene expression may be achieved by a trans-acting RNA-binding protein attaching to similar cis-elements in different, yet functionally clustered, mRNAs. The RNP granule can be regarded as a supramolecular assembly of RNA and protein, probably representing several overlapping post-transcriptional operons. The present protocol describes how RNP granules may be isolated by the transgenic expression of a 3X FLAG version of an RNA-binding protein under tetracycline control via the tetracycline receptor/operator complex. In this way, inclusion of an appropriate tetracycline concentration ensures expression of the tagged version at the endogenous level, and the 3X FLAG tag is a convenient "handle" for the subsequent immunoprecipitation by immobilized anti-FLAG antibody.