Scatterplot of across-tissue mean expression vs. across-tissue standard deviation reveals two classes of segments. Segments with three or more genes are plotted. The tissue-specific expression value of a segment is taken to be the average of its component genes’ values; the mean and standard deviation across tissues of each segment’s values are the coordinates for the plotted point. The horizontal dotted line denotes the cutoff for the top quartile of segments by standard deviation. The vertical dotted line denotes the median gene expression value across all genes and tissues. Segments close to the X-axis have similar expression values for every tissue; those close to the Y-axis have high expression in a minority of tissues.