Publisher Summary RNA tumor viruses, commonly called “oncornavirus,” are essentially categorized as type-A, type-B, type-C, and type-D, primarily on the basis of morphologic criteria. Type-C RNA tumor viruses are regarded as the most extensively studied class of oncornaviruses. This group of viruses is characterized by their centrally located nucleoid and a pattern of virion assembly that occurs as a budding process at the plasma membrane. This chapter focuses on type-C viruses and their translational products. It also presents RNA tumor viruses, essentially known as “RNA sarcoma viruses.” Type-C oncornaviruses can be distinguished from either type-B or type-D viruses on the basis of both morphologic criteria and the divalent cation preference of their RNA-dependent DNA-polymerase. A number of structural proteins of all type-C oncornavirus isolates essentially share cross-reactive interspecies antigenic determinants. The type-C virus genome consists of two identical 35 S RNA subunits joined near their 5′ terminus in a hydrogen-bonded dimer linkage structure. The genome is terminally redundant containing short identical segments at its 5′ and 3′ ends that provide a means of forming circular structures during replication. The mammalian type-C viral genome appears to be arranged in the order 5′gag-pol-env 3′. In the case of replication-defective mammalian sarcoma viruses, src sequences acquired from the host cell genome by genetic recombination are located at varying positions, frequently extending into the 3′terminus of the gag gene.