Publisher Summary This chapter defines and discusses the mapping of adenovirus 2 nuclear transcription. The immediate products of genes in mammalian cells are not used directly as mRNA but undergo extensive posttranscriptional modifications. At the level of transcription, the control of gene expression in mammalian cells may involve the posttranscriptional modifications. Therefore, any step concerned with processing of a nuclear precursor molecule can potentially be involved in the regulation of gene expression. Adenovirus-infected HeLa cells provide an ideal system for studying the formation of primary RNA transcripts. Adenovirus is an excellent model system for the study of the formation of primary transcripts because of the availability of highly refined physical maps of the adenovirus 2 genome that have been obtained using restriction endonucleases. This then allows a relationship to be established between DNA structure, mRNA location, and definition of transcription units responsible for mRNA production.