As the genome was complicated and compartmentalized during their origin and evolution, eukaryotes had to develop a system for local and highly coordinated expression of genetic material. At the first and one of the most important levels (transcription from DNA into RNA), this problem was solved by developing intricate transcription factories, which, like giant assembly plants, concentrated all factors regulating the gene expression and the fate of nascent mRNA (various proteins, many small nuclear RNAs, RNPs, etc.). To perceive and process the signals from all these various regulators, the eukaryotic transcription machinery utilizes the C-terminal domain (CTD) of Rpb1, the largest subunit of RNA polymerase II. The CTD acts as a highly sensitive, self-adjustable wide-range antenna. The review considers the structure, modification, interactions, and evolution of the CTD in the context of its role in coupling the transcription steps and mRNA maturation in eukaryotes. © Nauka/Interperiodica 2007.