ABSTRACT
All the cells of a given organism possess an identical genotype, the literal sequence content of their entire genome. The unique phenotypes (observable characteristics) displayed by neurons, osteoblasts, hepatocytes, etc., within an organism derive from differential gene expression, protein synthesis, and activation. Over the last several decades, numerous studies have demonstrated the critical and cooperative role of extracellular stimuli in the development and maintenance of cellular phenotypic characteristics and functions [1,2]. Growth factors, hormones, and extracellular matrix adhesion molecules have been shown to in”uence cell survival, division, migration, and differentiation [3,4]. In general, these effects are mediated through transmembrane receptors that couple extracellular ligand binding to the activation of intracellular enzymes. Subsequent signal transduction cascades of cytoplasmic enzymatic reactions ultimately culminate in the activation of transcription factors, proteins that bind to regulatory sequences within DNA, activating or inhibiting transcription of speci™c genes.
