The oncologist's debt to the chicken

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Abstract
Our understanding of avian tumour virology has laid the groundwork for much basic cancer research. Most outstanding has been the delineation of oncogenes. These were first discovered in avian leukosis and sarcoma viruses, but are of fundamental importance in most types of non‐viral cancer too, both in animals and humans. Avian tumour viruses gave us the tools to probe the complex signalling pathways in the cell that go awry in cancer. Avian retroviruses have also aided medical research in other ways: HIV infection is treated by anti‐retroviral drugs directed against enzymes, such as reverse transcriptase, first discovered in chicken retroviruses. Endogenous viral genomes are becoming linked to normal development, to disease and to concern over xenotransplantation; again, the chicken provided the initial knowledge. Retroviral vectors used to deliver human gene therapy have borrowed extensively from natural oncogenic and genetically‐engineered avian retroviruses. Marek's disease virus (MDV) has served as a model transforming lymphotropic herpesvirus for human lymphomas and lymphoproliferative disease caused by Epstein‐Barr virus. Unravelling a novel human herpesvirus causing Kaposi's sarcoma also owes a debt to the chicken, not only to MDV, but also to the chorioallantoic membrane assay as a model for tumour angiogenesis.