DNA release and uptake in an in vivo mammalian system is described for the first time, and is related to normal growth and differentiation, abnormal growth and cancer. It is proposed that there is a homeostatic exchange system of intercellular factors including genetic material (DNA/RNA) between cells and tissues, important for growth and differentiation. Disturbances of the messages or the homeostatic control of the exchange system can result in abnormal growth and cancer.