Abstract
During the past few years, it has become increasingly evident that the expression of eukaryotic genomes is far more complex than had been previously noted. The idea that the transcriptome is derived exclusively from protein-coding genes and some specific non-coding RNAs--such as snRNAs, snoRNAs, tRNAs or rRNAs--has been swept away by numerous studies indicating that RNA polymerase II can be found at almost any genomic location. Pervasive transcription is widespread and, far from being a futile process, has a crucial role in controlling gene expression and genomic plasticity. Here, we review recent findings that point to cryptic transcription as a fundamental component of the regulation of eukaryotic genomes.