Inspection of the genes with the strongest membership for M9, M15 and M16 (Fig. 4) suggested that these modules consisted of genes that are preferentially expressed in oligodendrocytes, astrocytes and neurons, respectively, relative to other cell types in the brain. For example, several of the genes in M9 are involved in myelination, including MAL, PLLP and PMP22. Genes in M15 include SLC1A3, a glial high-affinity glutamate transporter expressed in astrocytes. In M16, there are several genes with known functions in neuronal cells, including GABRG2 and SYN2. To test this hypothesis, we cross-referenced genes that serve as markers for specific cell classes in the adult mouse brain30,31 with all of the modules in each network in an unbiased manner. Comparisons with gene expression data generated from purified cellular populations30 revealed that M9, M15 and M16 were significantly enriched with markers of oligodendrocytes, astrocytes and neurons, respectively, in every network (P ≤ 4.3 × 10−37, P ≤ 2.5 × 10−61 and P ≤ 5.4 × 10−5, respectively; Table 1 and Supplementary Table 7 online). Comparisons with the results of genome-wide in situ
