Several modules were identified in some, but not all, brain regions. For example, M10 and M14, which showed properties related to glutamatergic synaptic function (Supplementary Note), were identified in CTX and CB, but not in CN. This finding is consistent with the predominant neuronal phenotype in each of these brain regions. Pyramidal neurons and granule neurons, which are the most abundant neurons in cerebral cortex and cerebellum, respectively, both release glutamate as their primary neurotransmitter. In contrast, the most abundant neuron in the caudate nucleus is the medium spiny neuron, which releases GABA as its primary neurotransmitter. Other modules with substantial overlap between brain regions suggest shared functional characteristics. For example, the overlap between M6A (CTX) and M6D (CB) implies that PVALB-positive cortical interneurons and Purkinje neurons may share similar elements of transcriptional programs related to inhibitory neurotransmission. It is also interesting to consider the absence of certain modules in specific brain regions. For example, it is not immediately clear why a module related to mitochondrial function (M7) was identified in CTX and CB, but not in CN, or why
