Although analogies are often made comparing development to cancer, there is of course a major difference. Normal development requires complex patterns of rigidly controlled cell proliferation and differentiation. In contrast, cancer represents the pathological condition that results when normal cell growth patterns are uncoupled from their regulatory influences. Genetic studies of RNA tumor viruses have provided insights into the relationships and differences of the genes responsible for normal development and cancer. The presence of discrete genes (oncogenes) within the genome of oncogenic retroviruses is responsible for their tumorigenic potential. Molecular genetic studies have found that normal eukaryotic cells possess genes that are quite homologous to the retroviral oncogenes. These normal cellular genes (proto-oncogenes) are involved in the regulation of proliferation and differentiation. However, if mutated, proto-oncogenes have the potential for inducing neoplastic transformation. The conversion of a proto-oncogene to an oncogene is called activation. Proto-oncogenes can become activated by a variety of genetic mechanisms including transduction, insertional mutagenesis, amplification, point mutations, and chromosomal translocations. In each instance the genetic aberration results in a proto-oncogene that is now free of its normal regulatory constraints. Such deregulation of function imparts a distinct growth advantage to the cell.