motor regions do have distinct whole-transcriptome signatures, probably related to their specialized cellular and functional architecture. It is also likely that other more subtle features of cortical parcellation may not have been detected in the current analysis, including those identified using neurotransmitter receptor distributions29 and functional connectivity30. One issue is that gyral patterns do not correlate perfectly with either cytoarchitectural or functional cortical parcellation. Greater regional differences may emerge if the samples can be grouped either by Brodmann area or on the basis of correlation to functional parcellations derived from functional imaging studies, now possible given the mapping of these data to MRI coordinates. Furthermore, it is likely that greater variation across areas will be found when assayed at the level of specific cortical cell types, as the excitatory neuron types in different layers display highly distinct molecular profiles31 that have been shown to vary significantly across areas in primate neocortex23. Finally, higher confidence in consistent regional differences should emerge as more samples are investigated32. Nevertheless, the relative homogeneity of the two largest neuronal structures, with ~69 billion (cerebellar cortex) and ~16 billion (cortex) neurons out of the 86 billion neurons in the human brain33, is striking and suggests an
