text: Arner et al. identified 65,423 transcribed enhancers (producing eRNA) among 33 different cell types under different conditions and different timings of stimulation. The transcription of enhancers generally preceded transcription of transcription factors which, in turn, generally preceded messenger RNA(mRNA) transcription of genes.
  Carullo et al. examined one particular cell type, neurons (from primary neuron cultures).  They exhibited 28,492 putative enhancers generating eRNAs. These eRNAs were often transcribed from both strands of the enhancer DNA in opposite directions.  Carullo et al. used these cultured neurons to examine the timing of specific enhancer eRNAs compared to the mRNAs of their target genes. The cultured neurons were activated and RNA was isolated from those neurons at 0, 3.75, 5, 7.5, 15, 30, and 60 minutes after activation. In these experimental conditions, they found that 2 of the 5 enhancers of the immediate early gene (IEG) FOS, that is FOS enhancer 1 and FOS enhancer 3, became activated and initiated transcription of their eRNAs (eRNA1 and eRNA3). FOS eRNA1 and eRNA3 were significantly up-regulated within 7.5 minutes, whereas FOS mRNA was only upregulated 15 minutes after stimulation.  Similar patterns occurred at IEGs FOSb and NR4A1, indicating that for many IEGs, eRNA induction precedes mRNA induction in response to neuronal activation.
  While some enhancers can activate their target promoters at their target genes without transcribing eRNA, most active enhancers do transcribe eRNA during activation of their target promoters.