The regulation of gene expression by specific signal transduction pathways
is closely connected to the cell phenotype. The response elicited by a given
transduction pathway varies according to the cell type. The finding that most
of the known nuclear oncogenes encode proteins involved in the regulation
of gene expression inspired the concept that the aberrant expression of some
key genes could cause cellular transformation or altered proliferation (Lewin
1991). The study, and ultimately the understanding, of these processes will
hopefully help us to unravel the profound changes that cause cancer and by
the same token the physiology of normal growth.