Paused RNA polymerase II as a developmental checkpoint.

The textbook view of gene activation is that the rate-limiting step is the interaction of RNA polymerase II (Pol II) with the gene's promoter. However, studies in a variety of systems, including human embryonic stem cells and the early Drosophila embryo, have begun to challenge this view. There is increasing evidence that differential gene expression often depends on the regulation of transcription elongation via the release of Pol II from the proximal promoter. I review the implications of this mechanism of gene activation with respect to the orderly unfolding of complex gene networks governing animal development.