The genes whose mutation contribute to the causation of cancer are known as cancer critical genes. These genes are grouped into two broad classes according to their mode of activity. For example, the genes that, after their mutation, gain hyperactivity of the gene product to cause cancer is protooncogene; their mutant, overactive form is called oncogene. The genes, which undergo a loss of functions due to mutation causing cancer, are called antioncogene or tumor suppressor genes. From the point of view of a cancer cell; oncogenes and tumor suppressor genes and their mutations that affect them are the two sides of the same coin.
These genes encode many kinds of proteins that help control cell growth and proliferation. Mutations in these genes can contribute to the development of cancer. Seven types of proteins that participate in controlling cell growth and the cancer cell result from expression of mutant form of these proteins. These are:
1. Growth factors, which function as ligands.
2. Growth factor receptors are membrane proteins that transduce the signal through its intracellular effector regions, often a tyrosine kinase. Sometimes ligands may act directly by intracellular receptors.
3. Signal transduction protein, which is an intracellular intransducer and mediated by second messengers.
4. Transcription factors, which act on DNA and initiate transcription of mRNA and then proteins are being synthesized.
5. Pro- and anti-apoptotic proteins direct programmed cell death.
6. Cell cycle control proteins.
7. DNA repair proteins, those are required for mismatch repair replication.
The mutations changing the structure or expression of proteins belonging to 1st-4th group generally give rise to dominantly active oncogenes. The protein belonging to 5th group acts as a tumor suppression, recessive mutation of these genes encoding proteins increase the surveillance of the cell and causing tumorous growth. Mutations in 7th group genes greatly increase the probability of mutations of the other group. A virus encoded growth receptor which is a miniature form of normal ligand receptor also can induce cancer.