One class of genes not described in this book is the oncogenes. It now seems fairly certain that many cancers are caused either by the malfunctioning of certain cellular genes or by a virus entering a host cell and so introducing additional genes able to transform normal cells into cancer cells. In recent years there have been great advances in understanding and defining these genes, which are termed oncogenes. Many of the oncogenes so far identified occur in the domestic fowl. Thus, in a review of oncogenes Bishop (1983) listed 25 of which nine were from the domestic fowl. At present more than 40 are known (Reddy, Skalka & Curran, 1988), and it is thought that the number may rise to the region of 100 (Watson et al., 1987), although the number of different classes appears to be less than 30. Since a number of the cellular oncogenes are present in normal uninfected cells they constitute part of the normal genome of the domestic fowl. Those oncogenes that are introduced on viral infection are related to their cellular counterparts. In this appendix the different types of oncogenes known to be present in the domestic fowl are listed, together with their protein products.