Unlike many animal viruses, infection by retroviruses generally does not lead to cessation of host RNA synthesis. Despite the high levels of host RNA in infected cells, the vast majority of retroviral particles contain a precise genomic complex consisting of two molecules of genomic RNA, rather than cellular or subgenomic viral mRNAs. Thus, the retroviral genome must be selected for encapsidation against a high background of cellular RNAs. It is therefore surprising that the retroviral genome is structurally similar to that of cellular mRNA. For instance, both molecules contain a 5′ m7 G cap and several hundred A residues at the 3′ terminus (reviewed in Coffin 1984a, 1985). Viral subgenomic mRNAs are even more similar to genomic RNA. The ability of the retroviral particle to choose correctly genomic RNA from the vast excess of heterologous molecules implies that specific sequences are present within the genome which direct the efficient encapsidation of the correct RNAs. Analysis of spontaneous and engineered mutants of both avian and murine retroviruses has in fact revealed that cis-acting sequences are involved and are present in the retroviral genome.