  Much of the genome is expressed in the vertebrate brain, with individual
genes exhibiting different spatially-varying patterns of expression. These
variations are not independent, with pairs of genes exhibiting complex patterns
of co-expression, such that two genes may be similarly expressed in one region,
but differentially expressed in other regions. These correlations have been
previously studied quantitatively, particularly for the gene expression atlas
of the mouse brain, but the biological meaning of the co-expression patterns
remains obscure. We propose a simple model of the co-expression patterns in
terms of spatial distributions of underlying cell types. We establish the
plausibility of the model in terms of a test set of cell types for which both
the gene expression profiles and the spatial distributions are known.
