1274 Genetic Transmission of Akr

Over the past 20 yr, evidence has progressively mounted that murine leukemia virus (MLV) ~ or its genetic determinants are ubiquitous in both highleukemic and low-leukemic strains of mice, and that viral genetic determinants are heritable components present in all of the cells (1-9). However, little is known of the state of the viral genetic determinants in the cells, i.e. if they are present as one or more D N A and/or RNA copies, and if they can be carried as elements of the host chromosome. In high-leukemic strains, such as AKR, infectious virus and viral antigens are present throughout life, while lowleukemic strains generally show little or no antigen or infectious virus until late in life. In some low-leukemic strains, virus-related antigens can be detected in embryos or in certain postnatal tissues in the absence of demonstrable infectious virus (10-13). The differences between inbred strains of mice in frequency and age when viral expression becomes detectable are sufficiently marked to allow genetic analysis of the transmission of viral antigen or infectivity.