Proto-oncogenes are common genes present. In all organisms and emerge to be highly conserved in the course of evolution. They have been isolated in species ranging from yeast to extremely evolved vertebrates including human being. This high degree of conservation implies their crucial role in the cell. Genes homologous to cellular proto- oncogenes are found in retroviruses recognized to cause cancer in various animal species. These viruses transform cells either by inserting their oncogenes in the host genome or by being present in numerous copies in the host cell. It is thought that the retroviruses picked up these oncogenes from metazoan cells they infected. As, their discovery more than 60 proto-oncogenes have been identified. Each has a role in mitosis. A number proto-oncogene encodes growth factors, proteins that stimulate cell proliferation. Several proto-oncogenes encode receptors for growth factors. A large number of them encode intracellular transducers, proteins which bring signals from cell receptors to nucleus, and transcription Factors which turn on gene transcription in the nucleus. The products of all proto-oncogenes work harmoniously at the right time in right amounts to induce cell division and differentiation.