also to new insights in cellular metabolism. The discoveries of cis-acting regulatory elements in transcription, the phenomenon of splicing of pre­ mRNA, and cap-dependent and cap-independent initiation of translation may be cited as examples. In addition, animal virus genomes contain elements and encode proteins that are very useful for the design of vectors for gene cloning and expression in mammalian cells. Apart from the basic interest in their biology, viruses have gained notoriety, of course, because they are pathogens. Human animal viruses may cause diseases ranging from the deadly (AIDS) to the benign (common cold). All studies on animal viruses potentially lead to the development of tools for their control, be it through prevention by immunization or treatment with antiviral drugs. Finally, viruses have yielded invaluable reagents in molecular biology as, for example, the vaccinia virus vector for the expression of foreign genes.