Adenovirus has contributed significantly to our current understanding of the organization and expression of genes in eukaryotic cells. The most startling discovery was probably the observation that most adenovirus mRNAs are encoded from discontinuous DNA segments. The split-gene and RNA-splicing concepts were rapidly extended to other systems and accepted as dogma within a few months after their initial discovery. It was clear right from the beginning that most adenovirus-transcription units encode multiple alternatively spliced mRNAs: Many are translated into proteins that have unique biological activities. It was also shown that the accumulation of alternatively spliced mRNA was temporally regulated during virus infection. The shift from the early to late pattern of mRNA accumulation was shown to be dependent on viral late-protein synthesis (1 ).