The transduction of signals from the extracellular space across the plasma membrane into the interior of cells and ultimately to the nucleus, where in - sponse to such external signals the transcription of the genetic code is inf- enced, belongs to the most fundamental and important events in the regulation of the life cycle of cells. During recent years several signal transduction cascades have been elucidated which regulate, for instance, the growth and the prolife- tion of organisms as diverse as mammals, flies, worms and yeast. The general picture which emerged from these investigations...