Location
It is common that IRESes are located in the 5'UTR of RNA viruses and allow translation of the RNAs in a cap-independent manner. However, mRNAs of viruses from Dicistroviridae family possess two ORFs (open reading frames), translation of the both being directed by two distinct IRESes. It was later suggested that some mammalian mRNAs also have IRES. Several cellular IRES elements are thought to be located in eukaryotic mRNAs encoding genes involved in stress survival, and other processes critical to survival. As of September 2009, there are 60 animal and 8 plant viruses reported to contain IRES segments and 115 mRNA sequences containing them as well.
Read more about this topic: Internal Ribosome Entry Site