In retroviruses, the "Gag" or core polyprotein is capable of assembling into virus particles and packaging the genomic RNA of the virus. How this protein recognizes viral RNA is not understood. Gag polyproteins contain a zinc-finger domain; mutants with changes in this domain assemble into virions, but a large fraction of these particles lack viral RNA. Thus, one crucial element in the RNA packaging mechanism is the zinc-finger domain. RNA sequences required for packaging ("packing signals") have been studied both by deletion analysis and by measuring encapsidation of nonviral mRNAs containing limited insertions of viral sequence. These experiments show that all or part of the packaging signal in viral RNA is located near the 5 end of the genome. These signals appear to be quite large, i.e., hundreds of nucleotides. Each virus particle actually contains a dimer of two identical, + strand genomic RNA molecules. The nature of the dimeric linkage is not understood. In some experimental situations (including zinc-finger mutants), only a small fraction of the particles in a virus preparation contain genomic RNA. It is striking that the genomic RNA packaged in these situations is dimeric. Because of this important observation, it is speculated that only dimers are packaged, and that the dimeric structure is an element of the packaging signal. It is also suggested that the dimers undergo a conformational change ("RNA maturation") after the virus is released from the cell, and that this change may depend upon the cleavage of the Gag polyprotein, a post-assembly event catalyzed by the virus-coded protease.