Illustration of asymmetry for a hypothetical gene. A hypothetical gene that is differentially expressed between heart and brain (highly expressed in heart). Y-axis gives the expression level on the log-scale. X-axis is the proportion of brain RNA making up the mixture (heart RNA makes up the remainder). Highlighted with the dotted lines are the mixtures with 5%, 25%, 75% and 95%, showing that finding differences between 5% and 25% (a 20% difference) and 75% and 95% (also a 20% difference) have very different degrees of difficulty.