Hamster polyomavirus (HaPV) is a small, circular DNA virus which belongs to the papovavirus family. The sequence of the viral genome is closely related to murine polyomavirus. The early protein coding region codes for three proteins expressed directly after infection. These proteins are responsible for the regulation of virus replication and for the transforming functions of the virus. The late protein coding region codes for the three viral structural proteins required for encapsidation of the viral genome in progeny. The structural proteins are only expressed at high levels in productively infected cells. Sequences essential for the regulation of viral gene transcription and viral DNA replication are in the virus noncoding region which separates the early and late open reading frames. HaPV was originally isolated from skin epitheliomas originating from hair follicle epithelial cells in Syrian golden hamsters; however, in certain strains of hamsters, HaPV infection also causes lymphomas. Two of the primary goals of ongoing studies are to establish a cell culture system to analyze productive viral replication and to analyze what viral and cellular proteins are involved in the immortalization and transformation of primary cell lines.