Distribution of microarray foreground signal values according to GST classes. In the five top panels, the graphs represent the number of microarrays experiments (out of a total of 276 hybridizations) in which a given probe had a statistically significant foreground signal. One bar corresponded to one GST feature that was ordered from left to right according to that number. The size (n) of each class is indicated in the top left corner of the five top graphs. The microarray design used for all experiments was registered as A-MEXP-58 in the ArrayExpress repository [17,18]. The graph in the bottom panel presents the curve overlay after scaling the X axis to 100% of the probes in each class. This graph also shows a cumulative distribution of the Cy3 channel signal of the Lucidea  background control spikes present on the microarray.