The increasing complexity of growth control pathways has been paralleled by the increasingly diverse functions uncovered for oncoproteins and tumor suppressor proteins. Our awareness of the interconnections between growth factor-and growth inhibitor-initiated signaling pathways has furthered our understanding of mechanisms underlying malignant transformation, but at the same time has indicated how complex cell growth control networks will be. In the past few years, there has been enormous progress in elucidating the details of mitogenic signaling. The positioning of multiple oncoproteins and tumor suppressor proteins on the same pathway has underscored the importance of some of these signaling pathways in transformation. For instance, ErbB, Ras, and Raf all lie on the ERK MAP kinase (MAPK) pathway, which through phosphorylation of members of the Ets protein family leads to induction of immediate early genes like c-fos.

The functions of oncoproteins and tumor suppressor proteins have been more than adequately reviewed in several places in the past few years. Taking this into account, and given the space constraints, I have chosen to discuss examples of more recently discovered oncoproteins and tumor suppressor proteins that illustrate new principles of cell regulation, emphasizing how such principles allow us to establish a regulatory network. The repertoire of cell surface receptor types involved in oncogenesis has grown, and I begin with an overview of several types of receptor-driven pathways implicated in oncogenesis. I then trace oncoprotein-activated signaling pathways to the nucleus and discuss the intriguing connections between the actin cytoskeleton and transformation. I end by reviewing the role that oncoproteins play in triggering cell cycle progression and preventing apoptosis.