Proto-oncogenes are usually responsible in making proteins which are important in stimulating division of cells, in stopping apoptosis or cell death, and in controlling cell differentiation. These processes are often necessary in the growth and maintenance of body organs and tissues. During embryogenesis, the action of these proto-oncogenes are mostly increased since tissues and organs are continually developing. As their functions are completed, some of these activities are often turned off.
There are several classifications of proto-oncogene groups, and these classifications are generally based on their function inside the cell. Examples of proto-oncogenes are receptor tyrosine kinases, growth factors, and membrane associated G-proteins, among many others.
From cell growth, through differentiation and during proliferation, these different kinds of proto-oncogenes are usually involved in the process. When these proto-oncogenes undergo mutations, however, they become oncogenes, which are capable of turning normal cells into cancer cells. Mutations are permanent alterations or changes that occur in the sequence of DNA in a given gene, often resulting in the production of a protein that functions differently, like increasing its activity or performance.