It is expected that gene expression profiles would be divergent between blood and brain tissues but similar for two brain regions. To test this, we ranked as percentiles the normalized gene expression values averaged for all subjects, setting non-detected probes to zero. Gene expression values were shown to be highly divergent between blood and either frontal cortex or cerebellum tissue for a large number of genes that were only detected reliably in one tissue or the other (Figs. 2A,B). In contrast, gene expression was more similar between frontal cortex and cerebellum and there were fewer uniquely expressed genes (Fig. 2C). Analysis using percentile ranked variance rather than mean values for each probe yielded similar results (Figs. 2D–F), showing that mean expression and variance in expression were closer in the two brain regions than in blood.
