The development of recombinant DNA technology has made a marked impact on molecular virology. The cleavage of viral DNA genomes with restriction enzymes and the cloning of such DNA fragments in bacterial p1asmids has led to the amplification of selected viral DNA fragments for sequencing and gene expression. RNA virus genomes which can be transcribed to their cDNA form were also cloned in bacterial p1asmids, facilitating the study of RNA virus genes. With the elucidation in recent years of the promoter sequence of various viral genes and the expression of these genes in bacteria or yeast, the...