Alternatively, the gene fusion resulting from a translocation may place one gene under the transcriptional control of another. In Burkitt's lymphoma, the myc gene becomes closely juxtaposed with transcription-regulating segments from one of the genome's three immunoglobulin gene clusters. The immunoglobulin gene then takes control from the normal myc gene transcriptional promoter and causes steady, unrelenting myc gene expression. The effects of oncogenes on the behavior of cells are best studied in cells grown in culture. Oncogenes can be introduced into cells by using either viral vectors or standard gene-transfer techniques.
Cells respond to introduced oncogenes in a variety of ways.
These responses in vitro appear to reflect related changes in cellular behavior occurring in a tumor mass in vivo. The most obvious response is a loss of growth control in monolayer culture. Normal cells will fill the bottom of a Petri dish and then cease further proliferation when they encounter one another. This response, known as contact inhibition, is abrogated in many cells that have been transformed by oncogenes. As a consequence, many oncogene-transformed cells will grow on top of one another, yielding easily observed clumps of transformants called foci.