Genetic variability can also result in differences in translational efficiency through changes in the mRNA sequence or in the level or sequence of regulatory RNAs.33 Both modes can be queried through high-throughput transcriptomic sequencing, which enumerates the number of times that any individual RNA species is present in preparations from that tissue. Unlike chip technologies, such sequencing does not depend on the relevant RNA being represented on an array; it can also provide a survey of all RNA species, not just mRNA. The eventual goal of the recently announced Genotype-Tissue Expression (GTEx) project is to create a whole-body map of haplotypic expression so that any risk haplotype for any disease can be easily checked for its effect on genomewide and tissuewide RNA expression (Fig. 2).
