is currently incomplete as there are other epigenetic modifications, such as acetylation, that are not currently accessible to high throughput techniques, and many more methylation sites exist than assayed here. Whether genotypic differences influence other components of the epigenome is unknown. Finally, in a complex organ such as brain cellularity will confound analyses correlating measures that vary between cell types. Thus, CpG methylation to mRNA correlations pose a considerable challenge in heterogeneous tissues. In the context of human tissues this problem may be solved by selection of cell type by laser dissection or by culture and terminal differentiation of cohorts of pluripotent stem cells. Both solutions have considerable technical limitations at this time.
