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FIGURE 4.1 A schematic diagram of a simple genetic network, consisting of four component modules, signified by ovals and circles (with the component, interacting genes not indicated). Each module’s expression is evoked by an external signal that can be one derived external to the cell (environmental, cell matrix-derived, hormonal, etc.) or by an internal gene product from another genetic module. Three of the modules depicted here have outputs in the form of target gene sets including cytodifferentiation gene products (A, D, and C) whereas one module (B) is purely regulatory. (Adapted from Wilkins, 2007b.)

Finally, there is the modular nature of the enhancer and silencer units that control whether and where and how long a particular gene product is expressed (reviewed in Davidson, 2001, 2006; see also Prud’homme et al., 2007, Chapter 6, this volume).

Mutational events that affect either the structure or the presence/ absence of particular domains within the encoded gene product or that affect the transcriptional modular units must influence how the respective genes are used in particular recruitment events. An example is the use of alternative v domains in the adhesion protein CD44 (Ruiz et al., 1995),), which can dramatically alter specific cellular capabilities of the expressing cells. The ubiquity of alternative splicing as a source of functionally altered proteins (Xing and Lee, 2006), however, serves as a general indicator of the importance of alterations in domain structure and composition as an input to regulatory change (Alonso and Wilkins, 2005). With respect to



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