Normal cells have strict growth control. They only in a specific and exact number of times with exact timing, depending on their location in the body. The so called Contact inhibition of normal cells prevents them from overlapping with each other. When normal cells touch each other, contact inhibition then stops the cells from dividing further.

Normal cells have a receptor protein in the membrane that regulates cell growth. Receptors such as insulin receptors, epidermal growth factor receptors and platelet derived growth factor receptors are bounded by growth factors to regulate growth and causes phosphorylation of tyrosine producing phosphotyrosine that acts as an intracellular growth messenger.

Malignant cells lose their contact inhibition and tend to pile up. They become immortal, dividing continuously as long as there is nutrient supply. In the human body, these overlapping growth of cells causes damage as well as imbalance in the different systems of the body.

The Rous sarcoma virus oncogene encodes a transmembrane protein that also phosphorylates tyrosine but at 10 - 20 times the normal rate. The erb-b oncogene that causes cancer in chickens encodes a protein similar to the epidermal growth factor. There are also oncogenes that encodes proteins similar to the platelet derived growth factors and insulin receptor.
Retroviridae are those viruses which affects blood cells and cells in the immune system causing leukemia and sarcoma. Some retroviruses cause cancer directly by integrating an intact oncogene into the DNA or indirectly through protooncogenes.

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