Internal ribosome entry sites in cellular mRNAs: Mystery of their existence

Although studies on viral gene expression were essential for the discovery of internal ribosome entry sites (IRESs), it is becoming increasingly clear that IRES activities are present in a significant number of cellular mRNAs. Remarkably, many of these IRES elements initiate translation of mRNAs encoding proteins that protect cells from stress (when the translation of the vast majority of cellular mRNAs is significantly impaired). The purpose of this review is to summarize the progress on the discovery and function of cellular IRESs. Recent findings on the structures of these IRESs and specifically regulation of their activity during nutritional stress, differentiation, and mitosis will be discussed.