Cancer genes by illegitimate recombination.

Retroviral onc genes are as yet the only proven cancer genes. They are generated by rare illegitimate recombinations between retroviruses and cellular genes, termed proto-onc genes. The claims that these proto-onc genes cause virus-free cancers upon "activation" by mechanisms that do not alter their germline structure are challenged. Instead, it is proposed that retroviral onc genes and cellular cancer genes are generated de novo by illegitimate recombinations that alter the germline structure of normal genes.