Th e body of published work is too large to be cited in this review -and it is likely to be only the tip of the iceberg, with a lot more unpublished data scattered through out public and private repositories. Other eff orts have yielded, for instance, blood transcriptional signatures in patients with neurodegenerative diseases [109] [110] [111] , and those associated with disease exacerbation or responsiveness to glucocorticoids in patients with asthma [112, 113] , and with responses to environmental exposure [114] [115] [116] , exercise [117, 118] or even laughter [119] . Unfortunately, too many published studies are underpowered and sometimes lack even the most rudimentary validation steps. All too often primary data are not available for reanalysis either, refl ecting a lack of enforcement of editorial policies, or the absence thereof in some journals. Hence, one of the main challenges for this fi eld is to move beyond the proof of principle stage and consolidate the wealth of data being generated.