Associations between expression level and gene-wise correlation and significance group. A) Two-dimensional density plots of expression value versus correlation in different significance groups. B) Two-dimensional density plots of Illumina expression value versus p-value ranks in different significance groups. The density plot for Affymetrix expression value versus p-value ranks is shown in Additional File 1. There are proportionately more genes in Gns in the low-expression group compared to the high expression group. The converse is true for Gai. The density plots were obtained via the MASS package in R . The bimodal distribution is most noticeable in Gns.