Retroviruses have been of great importance to biomedical science for the past half century. Initially, studies on oncogenic animal retroviruses provided important insights into molecular processes in carcinogenesis - most notably the existence and mechanisms of action of oncogenes and proto-oncogenes. Moreover, several human diseases are caused by retroviruses, including AIDS, adult T-cell leukemia and the neurological disease HAM/TSP. The topic of this volume is a relatively unknown animal retrovirus, jaagsiekte sheep retrovirus, the causative agent of transmissible lung cancer in sheep...