There is a substantial amount of evidence that multiple changes in gene expression occur throughout the process of cell differentiation. That is, as an organism develops from the undifferentiated ball of cells that result from a fertilized egg, to a developing embryo, to a multitude of cell and tissue types present in an adult, a single gene may express itself in multiple ways depending upon its developmental context, i.e. the particular stage of development of the cell it is in as well as millions of events external and internal to the cell. Similarly, many changes in gene expression have been associated with complex somatic diseases (Cookson, Liang, Abecasis, Moffatt, & Lathrop, 2009; Dermitzakis, 2008; Emilsson, et al., 2008), developmental and behavioral disorders (Buechel, et al., 2011; Gregg, et al., 2008; Mudge, et al., 2008), and age-related changes (e.g., Lu, Pan, Kao, Li, Kohane, Chan, et al., 2004; Somel, Franz, Yan, Lorenc, Guo, Giger, et al., 2009) in different tissue types and in the organism as a whole.
