The action of oncogenes in the cytoplasm and nucleus.

As many as 40 distinct oncogenes of viral and cellular origin have been identified to date. Many of these genes can be grouped into functional classes on the basis of their effects on cellular phenotype. These groupings suggest a small number of mechanisms of action of the oncogene-encoded proteins. Some data suggest that, in the cytoplasm, these proteins may regulate levels of critical second messenger molecules; in the nucleus, these proteins may modulate the activity of the cell's transcriptional machinery. Many of the gene products can also be related to a signaling pathway that determines the cell's response to growth-stimulating factors. Because some of these genes are expressed in nongrowing, differentiated cells, the encoded proteins may in certain tissues mediate functions that are unrelated to cellular growth control.