Studies carried out over the past four decades on factors controlling the susceptibility to viral leukemias in mice have shown the existence of specific controlling gene loci. Evidence from a number of studies using various mouse strains and different leukemia viruses has shown that the controlling genes may act either directly or indirectly. "In vitro" experiments have shown controlling effects directed by the prospective target cells themselves in which effective virus infection and/or and/or replication is inhibitied. From "in vivo" experiments it is also evident that susceptibility may be controlled through indirect means mediated by immune relationships and other factors involved in the basic regulations of hematopoiesis. The present paper presents a brief review of the various genes known to control susceptibility and resistance to murine leukemia viruses. Certain conflicts in the literature relating to the naming of particular specific genes by different laboratories is also discussed, and some minor revisions in nomenclature designed to resolve some of the existing confusion are proposed.