Up-regulation of X-linked gene expression possibly facilitates the lower-than-expected transcriptional variation. (a) Considering the gene expression level and transcriptional variation of the same gene in two different cell lines (of the same species), when the gene expression is twofold lower in one cell (E1) than another (E2), we calculated the noise ratio CV1/CV2 as one group (black curve after sorting and logarithmic transformation) and as random pairs (grey curve after sorting and logarithmic transformation). The number of different pairs is shown, which demonstrates that transcriptional variation is significantly reduced when gene expression is up-regulated. (b) Regression of the mean transcript abundance of each chromosome against the mean transcriptional noise of each chromosome. On a chromosomal level transcriptional noise is negatively correlated with gene abundance variation.