Not available – first paragraph follows: A remarkably simple genetic system for study of DNA multiplication and gene expression in plants is provided by DNA plant viruses. These viruses have only a half-dozen or so genes that are believed to be regulated in the same way as other plant genes. The DNA replicates in nuclei and may be associated with nuclear proteins (histones) in the same way as plant genetic material. Thus, the virus provides a small-scale, readily manipulated model for gene expression.