Expression patterns for the most differentially expressed 10% of these PSD genes between any pair of structures are displayed in Fig. 2b (74 genes with at least a 10.6-fold difference). The synapse-associated genes clustered into groups enriched in specific regions, indicative of a diverse set of excitatory synapse subtypes. For example, the primary motor cortex in the precentral gyrus, the origin of the longest range projection neurons, is delineated by selective enrichment of neurofilament proteins NEFL, NEFM and NEFH, which are frequently enriched in long-range projection neurons14. Surprisingly, a number of the most differentially expressed PSD-associated genes seem to be synthesized by glia, an observation made obvious by the stereotyped structural distribution of oligodendrocytes in white matter and other brain regions and the presence of well known myelin-associated genes (for example, myelin oligodendrocyte glycoprotein, MOG; myelin basic protein, MBP) in this gene cluster (Fig. 2b, front rows). The presence of these proteins in PSD preparations may represent a carry-over of glial fragments. Alternatively, they may be components of glutamatergic synapses between neurons and oligodendrocytes, which have been shown to share
