Brain tissue is characterized by a high level of gene expression; at least 30–50% of ~25,000 known protein coding genes (International Human Genome Sequencing Consortium, 2004) are expressed across all parts of the brain (Colantuoni, Purcell, Bouton, & Pevsner, 2000; Myers, et al., 2007). Moreover, the human brain has the highest level of gene expression compared with other mammals such as the mouse (Enard, et al., 2002; Lockhart & Barlow, 2001) and Homo sapiens’ closest primate relative (Caceres, et al., 2003; Enard, et al., 2002; Khaitovich, Muetzel, She, Lachmann, Hellmann, Dietzsch, et al., 2004). The results of several studies comparing transcriptomes in human and chimpanzee brains suggest that most genes differentially expressed in these species are up-regulated, or more highly expressed, in the human brain than vice versa (Caceres, et al., 2003; Khaitovich, Muetzel, She, Lachmann, Hellmann, Dietzsch, et al., 2004). In contrast, gene expression differences in other human and chimpanzee tissues, such as the heart and liver, are nearly identical in their numbers of up-(increased expression of one or more genes) and down-(decreased expression of one or more genes)
