The genome of higher animals contains genes called cOnc which are thought to be responsible for cancer when activated. Many oncogenic retroviruses also contain vOnc genes responsible for their transforming properties. cOnc sequences are homologous and result, in fact, from genetic recombinations between retroviruses lacking transforming genes and the animal genome. Onc genes encode transforming proteins which are biological intermediates of their carcinogenic property and may consist of protein kinases active on membrane and cytoskeleton constituents or nuclear proteins probably active on DNA. Modifications of cOnc genes (or oncogenes) due, in particular, to chromosomal rearrangements and occasional mutations have been demonstrated in an increasing number of human cancers.