Host proteins, including specific transcription factors with binding sites near transcription start sites, are necessary for this specificity (32 - 33). (fruitfly.org)
When mapped to the genome, they tell us not only about the level of expression for various genes, but also which transcription start sites were used. (massgenomics.org)
The current project iteration, FANTOM5, has mapped enhancers and transcription start sites in hundreds of primary human cell types. (massgenomics.org)