genes that are most specifically and consistently expressed in the same cell type should appear highly correlated in microarray data derived from complex tissue homogenates. For a module consisting of such genes, the module eigengene can be interpreted as a proxy for the relative number of relevant cells present in each sample. Furthermore, a gene’s membership for such a module can be seen as a measure of expression fidelity for that particular cell type relative to other cell types that are present in the sample. In this context, it is interesting to consider the potential sources of variation that may influence the quantities of specific cell types in different samples of brain tissue (Supplementary Note).
