Publisher Summary This chapter discusses the restriction of murine leukemia viruses by Fv-1—that is, a model for studying host genetic control of retroviral gene movement and leukemogenesis. The Fv-1 locus is known as a genetically determined host-resistant factor to murine viral leukemogenesis. With the development of an in vitro cell culture system to define the host range of different murine leukemia virus isolates, this genetic factor was found to have two kinds of alleles, Fv-1n and Fv-lb, which specify resistance to B-tropic and N-tropic viruses, respectively, in a dominant fashion. The locus has been mapped adjacent to the Gpd gene in chromosome 4 of the mouse. It is significant that all mouse strains, wild mice included, have an Fv-1 locus and that all naturally occurring or chemically induced endogenous ecotropic murine leukemia viruses are susceptible to restriction by one allele or another. The chapter summarizes the basic facts that are important for further investigation of the molecular mechanism of Fv-1 restriction.