Differences in growth properties are also readily apparent. Most normal cells placed in culture in the laboratory  grow until they form a la monolayer that completely covers the bottom surface of the culture dish.. Once this monolayer is formed cell division ceases. When tumor cells are placed in culture, they grow to much higher cell densities. Tumor cells grow in "piles" (or multilayers) and they continue to grow even when the surface of the culture dish is completely covered (as long a there are sufficient nutrients).

Tumor formation is initiated via the conversion of normal cellular genes, called protooncogenes, into oncogenes. This conversion is a result of mutation (a change in the DNA sequence). Only a small minority (somewhere around one hundred or so) of the tens of thousands of genes in a cell can be converted to oncogenes.

The biological function of protooncogenes is not to cause cancer. That is, these genes are not just "sitting there" waiting to cause cancer. Rather these genes encode proteins that are part of the signal transduction pathways which control normal cellular growth and differentiation. Converting a protooncogene to an oncogene results in "throwing a monkey wrench" into the cellular "machinery". The result is the altered growth properties that we characterize as oncogenic transformation.

For more information see my article "The Molecular Basis of Cancer" which appeared in the December 1998 issue of the Bridgewater Review.