Question: How are GRBs (Genomic Regulatory Blocks) defined?

Context:
Reporter insertions distal to zebrafish developmental regulatory genes pax6.1/2, rx3, id1, and fgf8 and miRNA genes mirn9-1 and mirn9-5 recapitulate the expression patterns of these genes even if located inside or beyond bystander genes, suggesting that the regulatory domain of a developmental regulatory gene can extend into and beyond adjacent transcriptional units. We termed these chromosomal segments genomic regulatory blocks (GRBs).
We present evidence that microsynteny has been retained to keep large arrays of highly conserved noncoding elements (HCNEs) intact. These arrays span key developmental regulatory genes, forming genomic regulatory blocks (GRBs)
We recently described GRBs in vertebrates, where most HCNEs function as enhancers and HCNE arrays specify complex expression programs of their target genes
Although gene order in hox and other gene clusters has long been known to be conserved because of shared regulatory sequences or overlapping transcriptional units, the chromosomal areas found through insertional hotspots contain only one or a few developmental regulatory genes as well as phylogenetically unrelated genes. We have termed these regions genomic regulatory blocks (GRBs), and show that they underlie the phenomenon of conserved synteny through all sequenced vertebrate genomes.
Genomic regulatory blocks (GRBs) are chromosomal regions spanned by highly conserved non-coding elements (HCNEs), most of which serve as regulatory inputs of one target gene in the region.
The target genes are most often transcription factors involved in embryonic development and differentiation.
GRBs often contain extensive gene deserts, as well as additional 'bystander' genes intertwined with HCNEs but whose expression and function are unrelated to those of the target gene. 
We show evidence that GRB target genes have properties that set them apart from their bystanders as well as other genes in the genome: longer CpG islands, a higher number and wider spacing of alternative transcription start sites, and a distinct composition of transcription factor binding sites in their core/proximal promoters.
GRB targets are genes with a number of unique features that are the likely cause of their ability to respond to regulatory inputs from very long distances
Genomic regulatory blocks are chromosomal regions spanned by long clusters of highly conserved noncoding elements devoted to long-range regulation of developmental genes, often immobilizing other, unrelated genes into long-lasting syntenic arrangements.
Specifically, we discuss the recently discovered genomic regulatory blocks which we argue are principal units of vertebrate genome evolution and serve as the foundations onto which evolutionary innovations are built through sequence evolution and insertion of new cis-regulatory elements.
We show that these LD blocks contain highly conserved noncoding elements and overlap with the genomic regulatory blocks of the transcription factor genes HHEX, SOX4, and IRX3. 
Using a comparative genomics approach to reconstruct the fate of genomic regulatory blocks (GRBs) and identify exonic remnants that have survived the disappearance of their host genes after whole-genome duplication (WGD) in teleosts, we discover a set of 38 candidate cis-regulatory coding exons (RCEs) with predicted target genes. 
They are therefore functionally related to a number of key protein coding developmental genes, that form genomic regulatory blocks (GRBs) with arrays of highly conserved non-coding elements (HCNEs) functioning as long-range enhancers that collaboratively regulate the expression of their target genes.
 In this paper, we therefore systematically investigate the regulatory landscape around conserved self-transcribed miRNAs (ST miRNAs), with their own known or computationally inferred promoters, by analyzing the hallmarks of GRB target genes
Based on these results we used both an elevated HCNE density in the genomic vicinity as well as the presence of a bivalent promoter to identify 29 putative GRB target miRNAs/miRNA clusters, over two-thirds of which are known to play a role during development and differentiation.
Furthermore these predictions include miRNAs of the miR-9 family, which are the only experimentally verified GRB target miRNAs.
A subset of the conserved miRNA loci we investigated exhibits typical characteristics of GRB target genes, which may partially explain their complex expression profiles during development.
Developmental genes are regulated by complex, distantly located cis-regulatory modules (CRMs), often forming genomic regulatory blocks (GRBs) that are conserved among vertebrates and among insects.
We have investigated GRBs associated with Iroquois homeobox genes in 39 metazoans. 
Integrating sequence conservation, gene expression data, gene function, epigenetic marks, and other genomic features, we provide extensive evidence that many conserved ancient linkages involve (1) the coordinated transcription of neighboring genes, or (2) genomic regulatory blocks (GRBs) in which transcriptional enhancers controlling developmental genes are contained within nearby bystander genes.
As most of them occur as clusters near potential target genes, the database is organized along two hierarchical levels: individual UCNEs and ultra-conserved genomic regulatory blocks (UGRBs). 
Detailed synteny maps between the human and other genomes are provided for all UGRBs.
Genomic regulatory blocks (GRBs) are chromosomal regions spanned by highly conserved non-coding elements (HCNEs), most of which serve as regulatory inputs of one target gene in the region.
Integrating sequence conservation, gene expression data, gene function, epigenetic marks, and other genomic features, we provide extensive evidence that many conserved ancient linkages involve (1) the coordinated transcription of neighboring genes, or (2) genomic regulatory blocks (GRBs) in which transcriptional enhancers controlling developmental genes are contained within nearby bystander genes.
We have termed these regions genomic regulatory blocks (GRBs), and show that they underlie the phenomenon of conserved synteny through all sequenced vertebrate genomes.
They are therefore functionally related to a number of key protein coding developmental genes, that form genomic regulatory blocks (GRBs) with arrays of highly conserved non-coding elements (HCNEs) functioning as long-range enhancers that collaboratively regulate the expression of their target genes.
These elements are organized as large clusters, so-called gene regulatory blocks (GRBs) around key developmental genes.
The Sonic Hedgehog (Shh) genomic regulatory block (GRB) is one of the best functionally characterized examples, with several discrete enhancers reported within its introns, vast upstream gene-free region and neighboring genes (Lmbr1 and Rnf32).
BACKGROUND: Genomic regulatory blocks (GRBs) are chromosomal regions spanned by highly conserved non-coding elements (HCNEs), most of which serve as regulatory inputs of one target gene in the region. 
These elements are organized as large clusters, so-called gene regulatory blocks (GRBs) around key developmental genes. 
Developmental genes are regulated by complex, distantly located cis-regulatory modules (CRMs), often forming genomic regulatory blocks (GRBs) that are conserved among vertebrates and among insects.

Answer: Genomic regulatory blocks (GRBs) are chromosomal regions spanned by highly conserved non-coding elements (HCNEs), most of which serve as regulatory inputs of one target gene in the region. The target genes are most often transcription factors involved in embryonic development and differentiation. GRBs often contain extensive gene deserts, as well as additional 'bystander' genes intertwined with HCNEs but whose expression and function are unrelated to those of the target gene.
Genomic regulatory blocks are chromosomal regions spanned by long clusters of highly conserved noncoding elements devoted to long-range regulation of developmental genes, often immobilizing other, unrelated genes into long-lasting syntenic arrangements. Integrating sequence conservation, gene expression data, gene function, epigenetic marks, and other genomic features, we provide extensive evidence that many conserved ancient linkages involve (1) the coordinated transcription of neighboring genes, or (2) genomic regulatory blocks (GRBs) in which transcriptional enhancers controlling developmental genes are contained within nearby bystander genes. Although gene order in hox and other gene clusters has long been known to be conserved because of shared regulatory sequences or overlapping transcriptional units, the chromosomal areas found through insertional hotspots contain only one or a few developmental regulatory genes as well as phylogenetically unrelated genes. In addition, we generated ChIP-seq data for key histone modifications in zebrafish embryos, which provided further evidence of putative GRBs in embryonic development.
Genomic regulatory blocks (GRBs) are chromosomal regions spanned by highly conserved non-coding elements (HCNEs), most of which serve as regulatory inputs of one target gene in the region.