The transcriptome, i.e. the whole set of RNA transcripts in a cell, is generally conceived as a system whose major function is to pass information encoded in the genome sequence to the realm of phenotypes that underlie physiological and pathological traits. This messenger paradigm justifies the current interest for the genetics of gene expression [1]–[6] which has been further enhanced by the numerous associations between genetic markers and diseases reported in recent genome-wide association studies (GWAS) and the expected relevance of genome wide expression (GWE) to characterize the biological basis of these associations [7]–[10]. However the variability of gene expression not only reflects genetic variation but depends on other factors as well such as environmental exposures [11], [12], metabolic conditions [13], ageing [14], [15] or gender [16]–[17].
