The primary objective of the present proposal is to elucidate the mechanisms underlying control of age-dependent expression of RNA tumor viruses in BALB/c mice, a problem central to comprehending the etiology of viral induced leukemia. Specifically, it is intended 1) to determine if the information contained within the integrated viral genome is differentially expressed in BALB/c mice as a function of age; 2) to determine whether the regulation of age-dependent viral gene expression resides at the level of transcription or at some latter step in the processing of genetic information; 3) to characterize the macromolecules involved in age-dependent expression of RNA c-type viruses in BALB/c mice. It is anticipated that this approach will more clearly define at the molecular level processes associated with age-dependent expression of endogenous virus. Such expression may be important not only in determining the occurrence of viral induced neoplasia, but it may in a broader sense represent a common mechanism underlying a variety of age-related degenerative diseases.