A fundamental tenet of biology is that the phenotype of an organism is ultimately determined by its complement of genes. In multicellular organisms, it is the regulated pattern of expression of genes which determines the proliferation and differentiation of individual cell lineages and hence establishes the adult phenotype. It is therefore no surprise that both neoplasia and many developmental pathologies involve lesions in the regulation of specific genes. For this reason, an understanding of how genes are regulated has become an area of intense interest in both medicine and biology.